Ccnd1 deficiency suppresses G1-S checkpoint dysregulation and aberrant proliferation of teratoma-susceptible gonocytes. Oct4::GFP transgenic M19-Ccnd1 WT and KO embryonic testes were sectioned and immunostained for (A) phospho-pRB1 or (C) KI67 (E13.5, E14.5, E15.5). GFP-positive germ cells, positive and negative for phospho-pRB1 or KI67-positive, were counted. Data are plotted as the percentage of germ cells positive for phospho-pRB1 or KI67 ± SEM (n = 4-14). †, P < 0.01; ‡, P < 0.001; #, P < 0.0001. (B, D) Confocal microscopy images of E15.5 Oct4::GFP transgenic M19-Ccnd1 WT and KO gonads immunostained for (B) phospho-pRB1 or (D) KI67. Small groups of phospho-pRB1 or KI67-positive germ cells are noted (arrowheads). Scale bar: 50 μm.