PROM1 and neoplasm: We detected 2% to 4% of CD133+ cells in multiple cell lines derived from one Brca1 tumor with characteristics similar to those found in CD44+/CD24- cells, including drug resistance, the ability to form spheroids with further 30% enrichment in CD133+ cells, expression of stem cell genes, and in vivo reconstitution of tumors with as few as 100 cells.