Homozygous ataxia (axJ) mice have reduced expression of ubiquitin-specific protease 14 (Usp14), resulting in severe neuromuscular defects and death by 2 months of age. Transgenic expression of Usp14 exclusively in the nervous system of axJ mice (axJ-Tg) prevents early lethality and restores motor system function to the axJ mice, enabling an analysis of the reproductive capabilities of Usp14-deficient mice. Although female axJ-Tg mice had a 75% reduction of Usp14 in the ovaries, they were able to produce normal litters. Ovary transfer experiments also demonstrated that the ovaries of axJ mice were capable of producing viable pups. In contrast, male axJ and axJ-Tg mice displayed a 50% reduction in testicular Usp14 levels and were infertile, indicating that Usp14 is required for development and function of the male reproductive system. Immunohistochemistry experiments showed that Usp14 is found in the redundant nuclear envelope and cytoplasmic droplet of epididymal spermatozoa. Analysis of axJ testes demonstrated a 50% reduction in testis weight, a 100-fold reduction in sperm number and the presence of abnormal spermatozoa in the epididymis. Histological examination of the Usp14-deficient testes revealed abnormal spermatogenesis and the presence of degenerating germ cells, indicating that Usp14 and the ubiquitin proteasome system are required for spermatid differentiation during spermiogenesis. © 2008 Elsevier Inc. All rights reserved.