is a heterogeneous population of mutant cells, all of which share some mutations, but vary in specific phenotype. A tumor hosts several types of stem cells, one optimal to the specific environment and other less successful lines. These secondary lines may be more successful in other environments, allowing the tumor to adapt, including adaptation to therapeutic intervention. If correct, this concept impacts cancer stem cell-specific treatment regimes. Such a hierarchy would complicate attempts to pinpoint the origin. Identification CSCs, now reported in most human tumors, are commonly identified and enriched using strategies for identifying normal stem cells that are similar across studies. These procedures include fluorescence-activated cell sorting (FACS), with antibodies directed at cell-surface markers and functional approaches including side population assay or Aldefluor assay. The CSC-enriched result is then implanted, at various doses, in immune-deficient mice to assess its tumor development capacity. This in vivo assay is called a limiting dilution assay. The tumor cell subsets that can initiate tumor development at low cell numbers are further tested for self-renewal capacity in serial tumor studies. CSCs can also be identified by efflux of incorporated Hoechst dyes via multidrug resistance (MDR) and ATP-binding cassette (ABC) Transporters. Another approach is sphere-forming assays. Many normal stem cells such as hematopoietic or stem cells from tissues, under special culture conditions, form three-dimensional spheres that can differentiate. As with normal stem cells, the CSCs isolated from brain or prostate tumors also have the ability to form anchor-independent spheres. Recent years have seen an advent of genetic approaches to identify cancer stem cells in experimental rodents. In such studies, following the induction of cancer (usually through the application of mutagens), a genetic cassette is activated resulting in the expression of an easily identifiable marker, for instance green fluorescent protein (GFP). This overcomes the limitations of traditional approaches (e.g. the classic Bromodeoxyuridine (BrdU) labeling technique has been used to identify slow-cycling cells in animals) as genetic approaches are cell cycle independent and can be used for in vivo pulse-chase labeling to identify quiescent/slow-cycling cells. This strategy, for instance, was instrumental for identifying the so-called Lgr5+ compartment as a cancer stem cell compartment in liver cancer and showing its potential as a viable therapeutic target. Heterogeneity (markers) CSCs heterogeneity is a pool of differentiated and undifferentiated tumour cells that are replenished by cells possessing both tumour and stem cell like properties and having phenotypic and metabolic heterogeneity