Brain tumors may arise from and contain cancer stem cells (CSCs) capable of self-renewal, proliferation, and differentiation that recapitulate the parent tumor. There is compelling evidence that some human brain tumors (e.g.glioblastoma)  are heterogenous tissue composed from tumor cells and small portion of cancer stem cells – tumor-initiating cells, which have a high tumorigenic potential and a low proliferation rate. By definition, stem cells have an indefinite lifetime and reproduce over long periods of time making them likely to accumulate mutations that could lead to genetic instability. Glioma cancer stem cells are phenotypically similar to the normal stem cells, they express CD133 gene and other genes characteristic of neural stem cells (e.g. nestin) and posses the self-renewal potential. Cancer stem cells derived from glioblastoma are capable recapitulate original polyclonal tumors when xenografted to nude mice. They are chemoresistant and radioresistant and therefore responsible for tumor progression and recurrence after conventional glioblastoma therapy. Cancer stem cells contribute to glioma radioresistance by an increase of DNA repair capacity through preferential activation of the DNA damage response checkpoints.