CD44 and neoplasm: Comparing tumor initiation capacity, we found that at least 100 CD44, CD24, CD133, ALDH, or SP single positive cells are required to initiate tumors in mouse xenograft models[29,39,40,47,51]; whereas fewer of CD44+/CD117+ cells, CD44+/CD24+/EpCAM+ cells, or CD117+/lineage- cells[28,31,52] were required to form tumors when combination of two or three markers was used to isolate cells.