Fig. 6. Illustrative model of cell cycle progression and proliferation in PGCs. Cycling Pin1^-/- PGCs (bottom) have a lower BrdU labeling index (red), but normal Ki67, phosphohistone H3 and apoptosis marker profiles. This indicates that Pin1^-/- PGCs have a prolonged cell cycle because of defective cell cycle progression (larger, hatched circle) rather than cell cycle arrest and apoptosis. The net effect of decreased proliferation is fewer cell divisions (represented by arrows) in Pin1-deficient PGCs in the same time period, resulting in fewer PGCs at the end of the proliferative phase on 13.5 dpc in the absence of Pin1.