A good article on cancer stem cells : "The stem cell theory of cancer proposes that a relatively small number of rare stem cells drive tumor formation and progression. The majority of cancer cells, cancer stem cell (CSC) theory advocates say, can't sustain the tumor nor establish it elsewhere in the body; only CSCs are tumorigenic and have a metastatic phenotype. The CSC theory has significant implications for cancer research and therapy and may explain why treatments focused on reducing tumor mass by removing proliferating cells fail to eliminate tumors. Or maybe not. Despite a large number of publications supporting the existence of stem cells in tumors including human blood-cell derived cancers and solid tumors of the brain, breast, colon, pancreas, prostate, and skin, the issue of CSCs remains in serious contention. ... I think that there are some cancers that do clearly follow a cancer stem cell model, but it will be more complicated than what's been presented so far ... The existence of stem cells in tumors has been invoked to explain why some cancers keep going, no matter what chemotherapies or immunotherapies are used. Stem cells usually cycle slowly and thus are relatively insensitive to treatments aimed at stopping cell replication. CSCs are more resistant to conventional cancer drugs not only due to quiescence relative to cancer cells but also because they are characterized by increased expression of antiapoptotic proteins and drug efflux transports. Additionally, according to recent findings, they rapidly change antigen expression, making them unlikely targets for immunotherapies that target cell surface proteins."