Similarly, cells sorted for CD133, a cell population enriched in tumor initiating cells [27-29], were further enriched for Msi1, in agreement with Msi1 as a stem cell or early progenitor cell marker , and consistent with its enrichment in CD133+ breast cancer “mammospheres” [24], and poorly differentiated oral squamous cell carcinomas [34]. This evidence concerns the gene PROM1 and breast cancer.