AbstractThe Niemann–Pick C1 (NPC1) gene encodes for a multiple membrane spanning protein, which regulates the trafficking of low‐density lipoprotein‐mediated endocytosed cholesterol. Mutation of the human NPC1 gene causes Niemann–Pick type C (NPC) disease. The Npc1NIH mice, a model of human NPC disease, bear a spontaneous mutation of the Npc1 gene, and are infertile. In this study, we have performed sperm analysis to search for the cause of male infertility in the Npc1NIH mouse. The number of cauda sperms in Npc1−/− mice was decreased roughly three‐and‐half‐fold of that in wild‐type mice. The decreased sperm number in Npc1−/− mice is due, at least in part, to partial arrest of spermatogenesis in the testes, as revealed by histological analysis. Compared to wild‐type sperm, Npc1−/− sperm displayed a high frequency of morphological abnormalities, including tailless heads and aberrant heads. In the in vitro fertilization (IVF) assay using cumulus‐intact eggs, Npc1−/− sperm failed to produce two‐cell embryos. In the IVF assay where zona‐free eggs were used, Npc1−/− sperm bound normally but could not fuse with the egg. Further analysis indicated that Npc1−/− sperms are drastically impaired in the binding to the egg zona pellucida, only 14% of the level of wild‐type sperm. Moreover, on Npc1−/− cauda sperm, one‐third of the total cyritestin protein was not proteolytically processed, while fertilin β was processed normally. Taken together, these results demonstrate that there are multiple defects in sperms from mice lacking a functional NPC1 protein, and these observed sperm defects may result in sterility. Mol. Reprod. Dev. © 2006 Wiley‐Liss, Inc.