Normal cells that give rise to brain tumors. (A) Stem cells and progenitors. Stem cells are normal cells that can self-renew and give rise to all the cell types within a tissue. Progenitors, which arise from stem cells, can also proliferate but can only give rise to restricted cell types. In the brain, lineage-restricted progenitors give rise to neurons, astrocytes, and oligodendrocytes. (B) Cells of origin. The cell of origin for a tumor is the normal cell type that can give rise to the tumor following a particular oncogenic mutation. Both stem cells and progenitors can serve as cells of origin for brain tumors.