A prolyl isomerase Pin1 deficient (Pin1(-/-)) male mice had severe testicular atrophy. We investigated the function of Pin1 in spermatogenesis by analyzing the Pin1(-/-) mice at reproductive age. Pin1(-/-) mice had less alpha PLZF positive spermatogonia (undifferentiated spermatogonia) than wild type (WT). Nevertheless, the Pin1(-/-) testis contained approximately the same number of GFR alpha 1 positive spermatogonia (SSCs in steady state) as the WT testis. Furthermore, degeneration of the spermatogenia appeared in seminiferous tubules of 10 months old Pin1(-/-) mouse testis, and abnormal shape GFR alpha 1 positive spermatogonia were observed. In Pin1(-/-) spermatogonia, the ratio of the phospho-histone H3 positive cells (mitotic cells) in GFR alpha 1-positive spermatogonia was higher than that of WT. These results suggest that Pin1 promotes the progression of the mitotic cell cycle of SSC in steady-state, which is required for the sperm production from SSCs. (C) 2018 Elsevier Inc. All rights reserved.