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The growth of many tumors has been suggested to depend on a subset of tumor cells with an extensive capacity for self-renewal, termed cancer stem cells or tumor-initiating cells (TICs). These cells are not necessarily abundant or proliferative, but because they are long-lived and often resistant to conventional therapies, they are thought to contribute to tumor resistance and recurrence. Therefore, identifying these cells and finding approaches to targeting them has become an important goal in cancer research. We recently identified a population of TICs in a mouse model of medulloblastoma, the most common malignant brain tumor in children. These cells, marked by expression of the cell surface antigen CD15, are capable of propagating tumors following transplantation, whereas CD15– cells from the same donors cannot. To gain insight into the molecular mechanisms underlying the increased tumorigenic …