RB1 and atrial fibrillation: Due to the close relationship among cell proliferation and cell cycle, we further examined the protein levels of cell cycle regulators in AF-MSCs that were cultured in normoxia or 1% O2 hypoxia condition, and found that p21 and the phosphorylation of Rb were downregulated, and also observed increased phosphorylation of AKT, MEK and ERK, which were found to be important during cell proliferation and survival responses to 1% O2 hypoxia (Figure 1d).