renal cell carcinoma ( rcc ) is the most common epithelial malignancy of human adult kidneys , accounting for 3% of all neoplasms . at 40% , rcc has the highest mortality . approximately 30% of patients with rcc will be diagnosed with metastatic disease , and 60% of these patients will die from aggressive disease and metastases . within europe , immunotherapies such as interferon ( inf)- and interleukin ( il)-2 have been used for decades in metastatic rcc treatment but have provided positive effects in only 10  20% of cases . for the past ten years , clinical studies have used targeted molecular therapies such as orally administered sorafenib , sunitinib , pazopanib and tivozanib ( receptor tyrosine kinase inhibitors ) , as well as mammalian target of rapamycin ( mtor ) inhibitors ( everolimus and temsirolimus ) for treating patients with rcc . however , continuous treatments with these drugs are associated with a high incidence of toxic effects and resistance . the targets of these agents are the signalling pathways responsible for angiogenesis and cancer - cell proliferation . the clinical response is very low , and five - year survival of patients with metastatic rcc is only 10% [ 7 - 9 ] . increasing evidence shows the existence of small populations of tumour - initiating cells ( tics ) or cancer stem cells ( cscs ) that reside in the vicinity of the heterogeneous mass of the tumour . tics have been identified in melanoma   and many solid tumours , including brain , breast , prostate , ovarian , colon , gastric , pancreatic , head and neck   and liver cancers . like normal stem cells , these specialized tumour regenerating cells share common properties and can be characterized by their ability to self - renew and their capacity to form serially transplantable tumours in immune - deficient mice . on the basis of different protein expression on the cell surface , cells such as cd133 , cd105 , ncam , selecting side population ( sp ) can be easily isolated and identified through aldehyde dehydrogenase 1 ( aldh1 ) and rhodamine 123 ( rh123 ) dye activity [ 20 - 23 ] ( fig . afterward , treatment failure results in patient relapse . currently , there are no efficient treatments for tics . however , a number of pre - clinical trials have been performed using different therapeutic approaches to halt the proliferation of tics . tics have been identified and isolated from rcc patients and cell lines using mesenchymal stem cell marker cd105 ( endoglin ) . to evaluate the tumorigenicity of the isolated cells , cd105 ( 1x10 ) and cd105 ( 1x10 ) cells these cells appear to be important in rcc since angiogenesis is critical for tumour growth and development of metastasis , and cd105 is a membrane glycoprotein that is highly expressed in activated endothelial cells associated with angiogenesis . cd105 protein is part of a receptor for tgf - b1 and tgf - b3 that promotes cell proliferation and differentiation . moreover , endoglin expression was inversely correlated with tumour - node - metastasis  ( t - n - m ) . thus , endoglin could be a marker of tumour neovascularization and provide prognostic information in rcc development . others have also identified tics using hoechst 33342 dye as epithelial side population ( sp ) in malignant rcc human kidney tissue . they were enriched for high proliferation in culture conditions and stem cell - like characteristics . additionally , spheres derived from culturing single cells of rcc cell lines were enriched for cancer stem - like cells . these sphere - forming cells  were able to self - renew in in vivo and in vitro models and presented higher expression of  stemness genes and resistance to chemotherapeutic agents and radiotherapy compared to monolayer - adherent cells . recently , huang et al .   reported cancer stem cell - like cells with sp phenotype using hoechst 33342 dye in human primary rcc cell lines . sorted sp populations have high expression of abcb1 , abcg2 and abcc1 proteins and are resistant to chemotherapy and radiotherapy . however , pre - treatment of sp cells with verapamil , an abc transporter inhibitor , reversed drug resistance , thereby demonstrating that the abc transporter could be responsible for drug resistance . moreover , cancer - testis antigen and hsp40 family member dnajb8 were important in maintaining the tic / csc phenotype of the sp population in rcc cells , and overexpression of dnajb8 increased the percentage of sp cells . cd133 has been investigated as a putative stem cell marker in many solid tumours , including rcc . a small population of cd133 cells have been found in rcc and characterized as renal resident progenitor cells [ 26 , 34 ] . therefore , a low number of cd133 cells in rcc might be an early event in stem cell differentiation and possibly in malignant transformation . in addition , when undifferentiated cd133 cells from rcc were subcutaneously transplanted alone into scid in mice , the mice did not give rise to tumour formation , suggesting cd133 + are not tics . however , when co - transplantated with renal carcinoma cells , cd133 progenitors fostered tumour engraftment , growth and development . the tumours formed in vivo demonstrated that cd133 derived endothelial cells were able to form functional vessels enriched in human hla class i connected with the mouse vasculature . the expression level of cd133 in rcc patients biopsies were not correlated with clinical pathology and prognostic significance . the chemokine receptor cxcr4 is a putative stem cell marker , and its increased expression has been recently reported in renal carcinoma . two rcc cell lines derived from the primary and metastatic site were used to demonstrate that high expression of cxcr4 is associated with a more tumorigenic cell line . rcc cell lines derived from the metastatic site were found enriched in cxcr4 cells and capable of forming larger spheres in vitro , generating more tumours in mice when compared with cxcr4 cells . these results demonstrated that cxcr4 cells as a tic subpopulation the differences in cxcr4 expression reveal differences in the tics content in rcc cell lines . moreover , cxcr4 cells exhibited greater resistance to tyrosine kinase inhibitors and expressed other  stemness associated genes . surgical biopsies of the rcc patients confirmed that the high expression of cxcr4 has significant prognostic value and therapeutic importance . the ability of stem cells to efflux dye such as rhodamine 123 ( rh123 ) can be used to analyse and isolate these cells with progenitor characteristics . based on rh123 staining , rcc cells were characterized as rh123 and rh123 cells in a recent study . serially transplanted rh123 cells into scid mice were able to form tumours in all cases ( 12/12 cases ) . in addition , the rh123 cells showed higher differentiating potential and increased survival capability against radiotherapy compared to the rh123 cells . this finding indicates that tics might exist in rh123 populations in rcc cells , which is opposite the results reported in other cancers . observed neural cell adhesion molecule- ncam as a putative marker for malignant renal stem/  progenitor in wilms tumour . the intracellular aldehyde dehydrogenase 1 ( aldh1 ) functions to catalyse the oxidation of aldehyde and maintain cellular homeostasis . recent studies have shown that normal and cancer cells with high aldh1 activity have great potential to function as tics in metastatic rcc cell lines . aldh1 activity analysis revealed that sp populations from metastatic rcc cell lines have higher levels of activity for aldh1 compared to nsp ( non - side population ) . aldh1 cells also showed higher sphere - forming ability , self - renewal , tumorigenic and expressed higher mrna levels of stem cell associated genes . in summary , cd105 , cxcr4 and aldh1 activity could be suitable targets for identifying tumour- initiating cell markers in rcc [ 24 , 25 , 43 ] . the term metastasis is used to indicate the successful dissemination of tumour cells from the primary tumour site to other parts of the body . the discovery of novel biomarkers for identifying ctcs in many human cancers has provided a novel way of understanding tumorigenesis at the cellular level . the mechanism of the formation of metastasis is comprised of three events : ( 1 ) a group of cancer cells detach from the primary tumour . ( 2 ) the ctss circulate to distant parts of the body via the bloodstream or lymphatic system and adapt to the new environment . ( 3 ) the cells then settle in the new location to proliferate and colonize , giving rise to a metastatic tumour . however , this process is highly inefficient , as numerous disseminated cells die during migration , and some remain dormant for several years . because metastasis occurs through ctcs , examining these cells could lead to future research directions and help realize the cells clinical potential in rcc treatment and diagnosis . the clinical relevance of disseminated ctcs in the blood stream of rcc patients remains a controversial subject , as the biology of these cells has been poorly understood . investigations in patients with different tumours have assessed the prognostic value of disseminated epithelial cells in bone marrow and peripheral blood detected by immunocytochemistry using anti - cytokeratin antibodies [ 46 - 48 ] . only a few studies in rcc patients have demonstrated the possibility of cytokeratin - positive ( ck+ ) cells as markers for ctcs investigated independently in peripheral blood and bone marrow . an increase in the number of ck+ cells in rcc patients correlates with advanced tumour stage in rcc . found two types of ctcs in the peripheral blood of rcc patients : cytokeratin ( ck+ ) expressing cells and cells without cytokeratin expression , which are large with tumour - like morphology . similar results have been reported in earlier studies . in one study conducted with non - metastatic rcc patients , no prognostic relevance was found for disseminated cytokeratin - positive ( ck+ ) cells in the bone marrow . however , in another study , buchner et al .   found the prognostic values of disseminated ck+ cells in the bone marrow of patients with metastatic rcc indicated that these cells played an important role in tumour spread in metastatic rcc . the researchers also found that immunocytochemical detection of these cells could be useful in the assessment of clinical outcomes in patients . however , further characterization of the cells is necessary to evaluate their malignant potential as a relevant therapeutic target for novel systematic therapy . recently , el - heiliebi et al . used a new blood filtration technique to distinguish circulating non - hematologic cells ( cnhcs ) as ctcs in the blood of rcc patients based on cytomorphological criteria . gene expression analysis has been widely adopted in clinical and laboratory research to discern genomic background and identify genes that might serve as a prognostic biomarker in metastatic rcc . identifying such genes could serve to delineate novel targets for the invention of specific anti - cancer drugs . in a recent study , gene expression profiling was performed by comparing microarrays on samples from metastatic and primary rcc patients with other patients normal kidney tissue . the majority of up - regulated genes in these sets were those responsible for dna replication , cell cycle control , apoptosis and cell mortality . for instance , genes from the minichromosome maintenance gene family ( mcm2 , mcm4 , mcm6 and mcm7 ) , aurka - coding for a kinase , involved cell cycle regulation and tumour metastasis progression , as well as fen1 in dna replication . other researchers used a different approach to analyse the gene expression profiles based on stepwise progression and metastasis in rcc . samples from three different progressive sites of rcc were compared  kidney tissue ( n ) , through early tumour stage ( t1 ) to distant metastasis ( m ) . caveolin 1 , annexin a4 and lysyl oxidase were continuously deregulated on progression , indicating that these genes might be important in driving the tumour toward increased malignancy and metastasis . in a study of 58 cases , vasselli et al . identified a gene signature between the primary tumour and one with metastasis rcc . forty - five genes were consistently up- or down - regulated and associated with survival using the cox proportional hazards model . in addition , vascular cell adhesion molecule 1 ( vcam-1 ) was revealed to be the most likely biomarker predicting survival of rcc patients . the higher expression for the vcam-1 gene was significantly associated with longer survival compared to low vcam-1 expression . others have reported higher expression of cyyr1 and ldb2 genes associated with disease - free survival with higher expression in tumours that metastasized after 24 months compared to tumours that metastasized earlier . in a different approach , two cell lines derived from a matched primary tumour and adrenal metastasis from the same rcc patients were analysed using similar technology . furthermore , those highly expressed genes were associated with growth factors , cell division , signal transduction and cell - cell adhesion function . analysed the gene expression of cells shedding from two rcc cell lines with different metastatic potential . however , only caveolin-1 , cd44 and 3-integrin expression were down regulated in cells shed from both cell lines . these expressions might be important in faster migration of the tumour cells and give shed cells a metastatic advantage . molecular differences between tumour subtypes of rcc have been largely unknown . in a study of 112 rcc and normal kidney samples , researchers distinguished the gene expression pattern between three major types of rcc : clear cell rcc ( ccrcc ) , papillary rcc ( prcc ) and chromophobe rcc ( chrcc ) . gene expression was widely correlated with rcc subtype . down these genes were responsible for metabolism ( gapd ) , angiogenesis ( angptl4 and vegf ) , cell adhesion ( col3a1 and fn1 ) and immune response ( ighg3 and hla - drbi ) . up - regulation was reported in genes for osteopontin ( spp1 ) and retinol binding protein 4 ( rbp4 ) . moreover , primary tumour gene expression delineated 12 such genes that were associated with tumour metastasis formation and patient survival . among them , the most significantly deregulated genes were the human high - mobility group gene ( hmga1 ) , mitochondrial dienoyl - coa reductase ( decr1 ) , gtp - binding protein ( ragb ) and genes belonging to the gene families col5a1 , slc13a3 , slc29a2 , igfbp3 and gucy2c . in summary , these finding could serve to discriminate clinically meaningful sub classification of rcc and expression pattern approaches to describe metastasis formation in various rcc sub - types . extracting relevant biological insight from such genome data sets remains a major challenge . in recent years , the gene set enrichment analysis ( gsea ) method has been purposed to incorporate knowledge from databases with gene expression data set . used the gsea approach to analyse the expression profile in ccrcc by comparing poorly and well - differentiated tumour tissue samples . using comparative analysis , genes in these sets were involved in cell motility , signalling , proliferation and metastasis and were gene products in the building of extracellular matrix and as cell surface markers . renal cell carcinoma ( rcc ) is the most common epithelial malignancy of human adult kidneys , accounting for 3% of all neoplasms . at 40% , rcc has the highest mortality . approximately 30% of patients with rcc will be diagnosed with metastatic disease , and 60% of these patients will die from aggressive disease and metastases . within europe , immunotherapies such as interferon ( inf)- and interleukin ( il)-2 have been used for decades in metastatic rcc treatment but have provided positive effects in only 10  20% of cases . for the past ten years , clinical studies have used targeted molecular therapies such as orally administered sorafenib , sunitinib , pazopanib and tivozanib ( receptor tyrosine kinase inhibitors ) , as well as mammalian target of rapamycin ( mtor ) inhibitors ( everolimus and temsirolimus ) for treating patients with rcc . however , continuous treatments with these drugs are associated with a high incidence of toxic effects and resistance . the targets of these agents are the signalling pathways responsible for angiogenesis and cancer - cell proliferation . the clinical response is very low , and five - year survival of patients with metastatic rcc is only 10% [ 7 - 9 ] . increasing evidence shows the existence of small populations of tumour - initiating cells ( tics ) or cancer stem cells ( cscs ) that reside in the vicinity of the heterogeneous mass of the tumour . tics have been identified in melanoma   and many solid tumours , including brain , breast , prostate , ovarian , colon , gastric , pancreatic , head and neck   and liver cancers . like normal stem cells , these specialized tumour regenerating cells share common properties and can be characterized by their ability to self - renew and their capacity to form serially transplantable tumours in immune - deficient mice . on the basis of different protein expression on the cell surface , cells such as cd133 , cd105 , ncam , selecting side population ( sp ) can be easily isolated and identified through aldehyde dehydrogenase 1 ( aldh1 ) and rhodamine 123 ( rh123 ) dye activity [ 20 - 23 ] ( fig . . however , a number of pre - clinical trials have been performed using different therapeutic approaches to halt the proliferation of tics . tics have been identified and isolated from rcc patients and cell lines using mesenchymal stem cell marker cd105 ( endoglin ) . to evaluate the tumorigenicity of the isolated cells , cd105 ( 1x10 ) and cd105 ( 1x10 ) these cells appear to be important in rcc since angiogenesis is critical for tumour growth and development of metastasis , and cd105 is a membrane glycoprotein that is highly expressed in activated endothelial cells associated with angiogenesis . cd105 protein is part of a receptor for tgf - b1 and tgf - b3 that promotes cell proliferation and differentiation . moreover , endoglin expression was inversely correlated with tumour - node - metastasis  ( t - n - m ) . thus , endoglin could be a marker of tumour neovascularization and provide prognostic information in rcc development . others have also identified tics using hoechst 33342 dye as epithelial side population ( sp ) in malignant rcc human kidney tissue . they were enriched for high proliferation in culture conditions and stem cell - like characteristics . additionally , spheres derived from culturing single cells of rcc cell lines were enriched for cancer stem - like cells . these sphere - forming cells  were able to self - renew in in vivo and in vitro models and presented higher expression of  stemness genes and resistance to chemotherapeutic agents and radiotherapy compared to monolayer - adherent cells . recently , huang et al .   reported cancer stem cell - like cells with sp phenotype using hoechst 33342 dye in human primary rcc cell lines . less than 5% sp population has been reported in all studied rcc cell lines . sorted sp populations have high expression of abcb1 , abcg2 and abcc1 proteins and are resistant to chemotherapy and radiotherapy . however , pre - treatment of sp cells with verapamil , an abc transporter inhibitor , reversed drug resistance , thereby demonstrating that the abc transporter could be responsible for drug resistance . moreover , cancer - testis antigen and hsp40 family member dnajb8 were important in maintaining the tic / csc phenotype of the sp population in rcc cells , and overexpression of dnajb8 increased the percentage of sp cells . cd133 has been investigated as a putative stem cell marker in many solid tumours , including rcc . a small population of cd133 cells have been found in rcc and characterized as renal resident progenitor cells [ 26 , 34 ] . therefore , a low number of cd133 cells in rcc might be an early event in stem cell differentiation and possibly in malignant transformation . in addition , when undifferentiated cd133 cells from rcc were subcutaneously transplanted alone into scid in mice , the mice did not give rise to tumour formation , suggesting cd133 + are not tics . however , when co - transplantated with renal carcinoma cells , cd133 progenitors fostered tumour engraftment , growth and development . the tumours formed in vivo demonstrated that cd133 derived endothelial cells were able to form functional vessels enriched in human hla class i connected with the mouse vasculature . the expression level of cd133 in rcc patients biopsies were not correlated with clinical pathology and prognostic significance . the chemokine receptor cxcr4 is a putative stem cell marker , and its increased expression has been recently reported in renal carcinoma . two rcc cell lines derived from the primary and metastatic site were used to demonstrate that high expression of cxcr4 is associated with a more tumorigenic cell line . rcc cell lines derived from the metastatic site were found enriched in cxcr4 cells and capable of forming larger spheres in vitro , generating more tumours in mice when compared with cxcr4 cells . these results demonstrated that cxcr4 cells as a tic subpopulation the differences in cxcr4 expression reveal differences in the tics content in rcc cell lines . moreover , cxcr4 cells exhibited greater resistance to tyrosine kinase inhibitors and expressed other  stemness associated genes . surgical biopsies of the rcc patients confirmed that the high expression of cxcr4 has significant prognostic value and therapeutic importance . the ability of stem cells to efflux dye such as rhodamine 123 ( rh123 ) can be used to analyse and isolate these cells with progenitor characteristics . based on rh123 staining , rcc cells were characterized as rh123 and rh123 cells in a recent study . serially transplanted rh123 cells into scid mice were able to form tumours in all cases ( 12/12 cases ) . in addition , the rh123 cells showed higher differentiating potential and increased survival capability against radiotherapy compared to the rh123 cells . this finding indicates that tics might exist in rh123 populations in rcc cells , which is opposite the results reported in other cancers . observed neural cell adhesion molecule- ncam as a putative marker for malignant renal stem/  progenitor in wilms tumour . the intracellular aldehyde dehydrogenase 1 ( aldh1 ) functions to catalyse the oxidation of aldehyde and maintain cellular homeostasis . recent studies have shown that normal and cancer cells with high aldh1 activity have great potential to function as tics in metastatic rcc cell lines . aldh1 activity analysis revealed that sp populations from metastatic rcc cell lines have higher levels of activity for aldh1 compared to nsp ( non - side population ) . aldh1 cells also showed higher sphere - forming ability , self - renewal , tumorigenic and expressed higher mrna levels of stem cell associated genes . in summary , cd105 , cxcr4 and aldh1 activity could be suitable targets for identifying tumour- initiating cell markers in rcc [ 24 , 25 , 43 ] . the term metastasis is used to indicate the successful dissemination of tumour cells from the primary tumour site to other parts of the body . the discovery of novel biomarkers for identifying ctcs in many human cancers has provided a novel way of understanding tumorigenesis at the cellular level . the mechanism of the formation of metastasis is comprised of three events : ( 1 ) a group of cancer cells detach from the primary tumour . ( 2 ) the ctss circulate to distant parts of the body via the bloodstream or lymphatic system and adapt to the new environment . ( 3 ) the cells then settle in the new location to proliferate and colonize , giving rise to a metastatic tumour . however , this process is highly inefficient , as numerous disseminated cells die during migration , and some remain dormant for several years . because metastasis occurs through ctcs , examining these cells could lead to future research directions and help realize the cells clinical potential in rcc treatment and diagnosis . the clinical relevance of disseminated ctcs in the blood stream of rcc patients remains a controversial subject , as the biology of these cells has been poorly understood . investigations in patients with different tumours have assessed the prognostic value of disseminated epithelial cells in bone marrow and peripheral blood detected by immunocytochemistry using anti - cytokeratin antibodies [ 46 - 48 ] . only a few studies in rcc patients have demonstrated the possibility of cytokeratin - positive ( ck+ ) cells as markers for ctcs investigated independently in peripheral blood and bone marrow . an increase in the number of ck+ cells in rcc patients correlates with advanced tumour stage in rcc . found two types of ctcs in the peripheral blood of rcc patients : cytokeratin ( ck+ ) expressing cells and cells without cytokeratin expression , which are large with tumour - like morphology . similar results have been reported in earlier studies . in one study conducted with non - metastatic rcc patients , no prognostic relevance was found for disseminated cytokeratin - positive ( ck+ ) cells in the bone marrow . however , in another study , buchner et al .   found the prognostic values of disseminated ck+ cells in the bone marrow of patients with metastatic rcc indicated that these cells played an important role in tumour spread in metastatic rcc . the researchers also found that immunocytochemical detection of these cells could be useful in the assessment of clinical outcomes in patients . however , further characterization of the cells is necessary to evaluate their malignant potential as a relevant therapeutic target for novel systematic therapy . recently , el - heiliebi et al . used a new blood filtration technique to distinguish circulating non - hematologic cells ( cnhcs ) as ctcs in the blood of rcc patients based on cytomorphological criteria . gene expression analysis has been widely adopted in clinical and laboratory research to discern genomic background and identify genes that might serve as a prognostic biomarker in metastatic rcc . identifying such genes could serve to delineate novel targets for the invention of specific anti - cancer drugs . in a recent study , gene expression profiling was performed by comparing microarrays on samples from metastatic and primary rcc patients with other patients normal kidney tissue . the majority of up - regulated genes in these sets were those responsible for dna replication , cell cycle control , apoptosis and cell mortality . for instance , genes from the minichromosome maintenance gene family ( mcm2 , mcm4 , mcm6 and mcm7 ) , aurka - coding for a kinase , involved cell cycle regulation and tumour metastasis progression , as well as fen1 in dna replication . other researchers used a different approach to analyse the gene expression profiles based on stepwise progression and metastasis in rcc . samples from three different progressive sites of rcc were compared  kidney tissue ( n ) , through early tumour stage ( t1 ) to distant metastasis ( m ) . caveolin 1 , annexin a4 and lysyl oxidase were continuously deregulated on progression , indicating that these genes might be important in driving the tumour toward increased malignancy and metastasis . in a study of 58 cases , vasselli et al . identified a gene signature between the primary tumour and one with metastasis rcc . forty - five genes were consistently up- or down - regulated and associated with survival using the cox proportional hazards model . in addition , vascular cell adhesion molecule 1 ( vcam-1 ) was revealed to be the most likely biomarker predicting survival of rcc patients . the higher expression for the vcam-1 gene others have reported higher expression of cyyr1 and ldb2 genes associated with disease - free survival with higher expression in tumours that metastasized after 24 months compared to tumours that metastasized earlier . in a different approach , two cell lines derived from a matched primary tumour and adrenal metastasis from the same rcc patients were analysed using similar technology . furthermore , those highly expressed genes were associated with growth factors , cell division , signal transduction and cell - cell adhesion function . analysed the gene expression of cells shedding from two rcc cell lines with different metastatic potential . however , only caveolin-1 , cd44 and 3-integrin expression were down regulated in cells shed from both cell lines . these expressions might be important in faster migration of the tumour cells and give shed cells a metastatic advantage . molecular differences between tumour subtypes of rcc have been largely unknown . in a study of 112 rcc and normal kidney samples , researchers distinguished the gene expression pattern between three major types of rcc : clear cell rcc ( ccrcc ) , papillary rcc ( prcc ) and chromophobe rcc ( chrcc ) . gene expression was widely correlated with rcc subtype . down these genes were responsible for metabolism ( gapd ) , angiogenesis ( angptl4 and vegf ) , cell adhesion ( col3a1 and fn1 ) and immune response ( ighg3 and hla - drbi ) . up - regulation was reported in genes for osteopontin ( spp1 ) and retinol binding protein 4 ( rbp4 ) . moreover , primary tumour gene expression delineated 12 such genes that were associated with tumour metastasis formation and patient survival . among them , the most significantly deregulated genes were the human high - mobility group gene ( hmga1 ) , mitochondrial dienoyl - coa reductase ( decr1 ) , gtp - binding protein ( ragb ) and genes belonging to the gene families col5a1 , slc13a3 , slc29a2 , igfbp3 and gucy2c . in summary , these finding could serve to discriminate clinically meaningful sub classification of rcc and expression pattern approaches to describe metastasis formation in various rcc sub - types . extracting relevant biological insight from such genome data sets remains a major challenge . in recent years , the gene set enrichment analysis ( gsea ) method has been purposed to incorporate knowledge from databases with gene expression data set . used the gsea approach to analyse the expression profile in ccrcc by comparing poorly and well - differentiated tumour tissue samples . using comparative analysis , genes in these sets were involved in cell motility , signalling , proliferation and metastasis and were gene products in the building of extracellular matrix and as cell surface markers . in conclusion , this short review of the current literature has shown that the activities of cd105 , cxcr4 and aldh1 represent the most prominent markers for identifying tumour - initiating cells in rcc . in addition , cells expressing these markers share many common characteristics of the tic / csc phenotype , having strong proliferation potential , self - renewal ability , resistance against conventional therapy and the formation of tumours in in vivo studies . gene expression profiling helps to define tics and might help develop targeted therapeutics with refined diagnostic data for rcc cure . therefore , additional confirmation is needed , and future studies might be helpful to identify relevance with the clinical response in rcc .
metastasis is a complex process that propagates cells from the primary or initial site of the cancer occurrence to distant parts of the body . 
 cancer cells break from the cancer site and circulate through the bloodstream or lymph vessels , allowing them to reach nearly all parts of the body . these circulating tumour cells ( ctcs ) contain specialized metastasis - initiating cells ( mics ) that reside in the biological heterogeneous primary tumour . 
 researchers have hypothesized that metastasis of renal cell carcinoma is initiated by circulation of mics in patients blood and bone marrow . 
 based on the cancer stem / progenitor cell concept of carcinogenesis , understanding the molecular phenotypes of metastasis - initiating cells ( mics ) in renal cancer could play a vital role in developing strategies for therapeutic interventions in renal cancer . 
 existence of mics among ctcs in renal carcinoma has not been proven in large scale . 
 however , some studies have reported that specialized markers are found on the surface of circulating cells from the primary tumour . in mice , 
 mics have been isolated from ctcs using such markers , which have then been transplanted into xenograft model to show whether they give rise to metastasis in different organs . 
 considering these findings , in this review we have attempted to summarize the studies connected with mics and their gene expression profiles that are responsible for metastasis in renal cancer .