 found that brain tumor stem cells (BTSCs) are stem cells that can be isolated from brain tumors by similar assays used for neuronal stem cells.In forming clonal spheres similar to neurospheres of neuronal stem cells, these BTSCs were able to differentiate into neurons, astrocytes and oligodendrocytes "in vitro", yet more importantly capable of initiating tumors at low cell concentrations, providing a self-renewal capacity.It was therefore proposed that a small population of BTSCs with such self-renewal capabilities were maintaining tumors in diseases such as leukemia and breast cancer.