Stem cell
A cell that can both copy itself indefinitely and produce the specialised cells of a tissue. Cancers may be sustained by a small population of stem-like cells that survive treatment and regrow the tumour.
Blood stem cells in the bone marrow make every red cell, white cell and platelet for life, which is why chemotherapy that damages them causes anaemia, infection risk and bleeding, and why a stem cell transplant can rescue a patient after very high-dose treatment. The cancer stem cell idea holds that tumours are hierarchies too, with a minority of self-renewing cells at the top that are more resistant to chemotherapy and radiation; it is well supported in leukaemia and debated in solid tumours. Drugs aimed at stem-like states (against Wnt, Notch and Hedgehog signalling) have been largely disappointing so far.