CD44 and neoplasm: These observations not only corroborate our earlier contention that the CD44+ PCa cell population harbors tumor-initiating cells, but also establish requirements and the importance of CD44 and CD44+ cells in PCa development, consistent with other studies implicating critical/functional roles of CD44 in various CSCs and tumor systems including leukemic stem cells [59], [60], colon CSCs [10], gastric CSCs [18], and ovarian tumors [61].