a Comparison of the cell cycle patterns of embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs). General cell cycle profiles of ESCs (mESCs and hESCs) and MEFs are shown. b Differences in the expression of cell cycle components in mESCs and MEFs. In contrast to MEFs, mESCs continuously express cyclin A/E but lack the expression of cyclin D. This allows ESCs to retain retinoblastoma (Rb) hyperphosphorylation throughout the cell cycle, resulting in a rather short G1 phase. c Expression pattern of HR-related proteins between mES cells and MEFs. In the entire cell cycle, HR proteins are highly expressed in ESCs but not in MEFs.