Hypoxia or HIF-TWIST increases stem cell properties via suppressing p21. (A) siRNA against E2A in normoxic cells decreases the expression of p21. (B) siRNA against p21 in normoxic cells increases cell growth. Normoxic cells were cultured 12 days after p21 knockdown in low-density culture and cell numbers were counted. (C) Normoxic cells at passage 6 were stably transfected with scrambled or p21 siRNA, followed by differentiation into osteoblasts, adipocytes and chondrocytes for 3 weeks, and achievements of differentiation were analyzed by staining with ARS, Oil Red O and Alcian Blue, respectively. OD values of ARS and Oil Red O were analyzed for quantifying osteoblast and adipocyte differentiation, respectively. Knockdown of p21 increases the differentiation potential to osteoblasts, adipocytes and chondrocytes. Bar = 50 μm.