The testicular haploid expressed gene (Theg) encodes for a novel approximately 42.0-kDa nuclear protein, which 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. For functional dissection of the domain structure of the Theg protein, two different knockout approaches were undertaken. In the first knockout mouse (Th14), the C-terminal region of the Theg protein (amino acids 137-376) was deleted. Both Th14+/- and Th14-/- mice from genetic backgrounds of C57BL/6J x 129X1/SvJ hybrid and 129X1/SvJ inbred exhibited a normal phenotype and were fertile. The testes of Th14-/- mice were smaller than those of Th14+/- and Th14+/+ mice; however, the testicular morphology and the properties of sperm, including morphology and motility, from Th14-/- mice were similar to those of Th14+/- and Th14+/+ mice. These results demonstrate that the C-terminal region of Theg (amino acids 137-376) does not play an important role in progression of spermatogenesis. In the second knockout mouse (Th15), we deleted the N-terminal domain of the Theg protein, which resulted in complete loss of Theg transcripts. Both Th15+/- and Th15-/- mice from genetic backgrounds C57BL/6J x 129X1/SvJ hybrid, C3H/J congenic, and 129X1/SvJ inbred appeared normal and were fertile, with no gross abnormalities detected in testicular morphology or sperm properties. Our results from both knockout mouse model systems clearly illustrate that Theg is not essential for spermatogenesis in the mouse.