Stem cells, as functionally defined by McCullough & Till (1961-1964), while demonstrating the existence of the hematological stem cells predicted by LG Lajtha (1958), are cells that (1) can make more of themselves, (2)can undergo unlimited divisions (are immortal), and (3) can differentiate. For cancers, it has been proposed that, like normal heirarchical tissues, they also are maintained by a stem cell population (Cancer Stem Cells, CSCs; also called Tumor Initiating Cells TICs). These are the cells that must be eliminated for a cure.

Normal stem cells are also used (in some cases) where there is too much tumor to be able to eradicate it with conventional surgery plus radio- and/or chemotherapy, so massive doses of radiation or chemotherapy are given to ablate the patient's bone marrow and the cancer within it, and then hematological stem cells (preferably from an identical twin or immunologically matched donor) are injected to repopulate the bone marrow without the tumor.

An excellent chapter: Stem Cell Concepts, by L.G. Lajtha can be found in the book "Stem Cells: their identification and characterization", edited by CS Potten, and published by Churchill Livingston, London, 1983. This is chapter 1. My own chapter, "Stem cells in planarians", chapter 3 in that book, explains stem cell:tissue relationships in a system with morphologically identifiable totipotential stem cells.