Double-mutant C57BL/6J germ cells undergo mitotic arrest prior to M-phase. E17.5 double-mutant gonads from mixed and C57BL/6J backgrounds are compared with each other and with controls. (A,B) Control germ cells (red – note the different germ cell marker between mixed and C57BL/6J) are negative for Ki67 (green) because they have exited the cell cycle and are arrested in G0. (C,D) Double-mutant germ cells (red) on both mixed and C57BL/6J backgrounds remain in cycle and are Ki67-positive (green). (E,F) Control germ cells (red) are negative for pHH3 (green) because they have exited the cell cycle and are not undergoing active mitoses. (G,H) Double-mutant germ cells (red) on a mixed background (G) remain pHH3-positive, whereas double-mutant germ cells on a C57BL/6J background (H) are negative. This indicates that mixed-background, double-mutant germ cells continue to divide, whereas those on a C57BL/6J background successfully arrest prior to M-phase. ECAD is a membrane marker and labels germ cells in the mixed background. Oct4-EGFP is both nuclear and cytoplasmic and labels germ cells in the C57BL/6J background. Scale bars: 50 μm.