The knock-down of miR-21 expression by its siRNA inhibitor decreased the expression of VEGF, CD44, EpCAM, and the CSC self-renewal capacity of PCa sphere-forming cells under hypoxic condition, which suggest that hypoxia-induced miR-21 expression plays a critical role within the tumor microenvironment, contributing to tumor aggressiveness. This evidence concerns the gene CD44 and neoplasm.