inside the single tumour mass. There are two theories to explain the phenotypic and metabolic heterogeneity of CSCs; clonal variation and cancer stem cell theory. While former theory dictates the role of genetic, epigenetic and micro environment where tumour cell resides to acquire undifferentiated tumorigenic traits. The latter theory focus more on the malignancy traits acquired by stem cells where these undifferentiated and highly tumorigenic stem cells repopulate the differentiated tumour mass. CSCs have been identified in various solid tumors. Commonly, markers specific for normal stem cells are used for isolating CSCs from solid and hematological tumors. Markers most frequently used for CSC isolation include: CD133 (also known as PROM1), CD44, ALDH1A1, CD34, CD24 and EpCAM (epithelial cell adhesion molecule, also known as epithelial specific antigen, ESA). CD133 (prominin 1) is a five-transmembrane domain glycoprotein expressed on CD34+ stem and progenitor cells, in endothelial precursors and fetal neural stem cells. It has been detected using its glycosylated epitope known as AC133. EpCAM (epithelial cell adhesion molecule, ESA, TROP1) is hemophilic Ca2+-independent cell adhesion molecule expressed on the basolateral surface of most epithelial cells. CD90 (THY1) is a glycosylphosphatidylinositol glycoprotein anchored in the plasma membrane and involved in signal transduction. It may also mediate adhesion between thymocytes and thymic stroma. CD44 (PGP1) is an adhesion molecule that has pleiotropic roles in cell signaling, migration and homing. It has multiple isoforms, including CD44H, which exhibits high affinity for hyaluronate and CD44V which has metastatic properties. CD24 (HSA) is a glycosylated glycosylphosphatidylinositol-anchored adhesion molecule, which has co-stimulatory role in B and T cells. CD200 (OX-2) is a type 1 membrane glycoprotein, which delivers an inhibitory signal to immune cells including T cells, natural killer cells and macrophages. Leptin receptor (LEPR) is a type I cytokine receptor for the fat associate hormone leptin, which plays an important role in promoting malignant progression. ALDH is a ubiquitous aldehyde dehydrogenase family of enzymes, which catalyzes the oxidation of aromatic aldehydes to carboxyl acids. For instance, it has a role in conversion of retinol to retinoic acid, which is essential for survival. The first solid malignancy from which CSCs were isolated and identified was breast cancer and they are the most intensely studied. Breast CSCs have been enriched in CD44+CD24−/low, SP and ALDH+ subpopulations. Breast CSCs are apparently phenotypically diverse. CSC marker expression in breast cancer cells is apparently heterogeneous and breast CSC populations vary across tumors.