Subpopulations of tumorigenic cells have been identified in many human tumors, although these cells may not be very rare in some types of cancer. Here, we report that medulloblastomas arising from Patched-1–deficient mice contain a subpopulation of cells that show a neural precursor phenotype, clonogenic and multilineage differentiation capacity, activated Hedgehog signaling, wild-type Patched-1 expression, and the ability to initiate tumors following allogeneic orthotopic transplantation. The normal neural stem cell surface antigen CD15 enriches for the in vitro proliferative and in vivo tumorigenic potential from uncultured medulloblastomas, supporting the existence of a cancer stem cell hierarchy in this clinically relevant mouse model of cancer. [Cancer Res 2009;69(11):4682–90]