Stage-specific interactions between Sertoli cells and germ cells plays central role in the regulation of spermatogenesis. In the first part of the study, function of Theg (testicular haploid expressed gene) gene was characterized. Theg is specifically expressed in spermatid cells. Its expression is upregulated by some unknown factor(s) from Sertoli cells. To elucidate the function of THEG protein and its role in spermatogenesis, we disrupted the Theg locus in mouse by homologous recombination. The Theg knockout mice from two different genetic backgrounds appeared normal and were fertile, with no gross abnormalities detected in testicular morphology or sperm properties. Our results from knockout mouse model systems clearly illustrate that Theg is not essential for spermatogenesis in the mouse. During the generation Theg knockout mice, an intriguing mutation was discovered, which we named as nax. The nax mice exhibit delayed hair appearance and ataxia in a homozygote state. Histological analyses of nax brain revealed an overall impairment of the cerebellar cortex. We mapped the nax locus between marker D2Mit158 and D2Mit100 within a region of 800 kb in the middle of chromosome 2.