Spermatogenesis is a developmental process that occurs in several phases and is regulated by a large number of gene products. An insertional transgenic mouse mutant (termed kisimo mouse) has been isolated that results in abnormal germ-cell development, showing abnormal elongated spermatids in the lumina of seminiferous tubules. We cloned the disrupted locus of kisimo and identified a novel testis-specific gene, THEG, which is specifically expressed in spermatids and was disrupted in the transgenic mouse. The yeast two-hybrid screening method revealed that THEG protein strongly interacts with chaperonin containing t-complex polypeptide-1⑀, suggesting that THEG protein functions as a regulatory factor in protein assembly. Our findings indicate that the kisimo locus is essential for the maintenance of spermiogenesis and that a gene expression disorder may be involved in male infertility.The integration of foreign DNA into the mouse germ line by retroviral infection or microinjection can result in insertional disruption of endogenous genes with important roles in development (1-3). Foreign DNA insertion provides an approach to the cloning of disrupted host loci. By using the introduced DNA as a probe to screen genomic libraries from mutant animals, it has been possible in a few instances to isolate clones that contain DNA flanking the exogenous integrated material and, thus, include portions of the interrupted gene (4, 5). We have produced a series of 12 transgenic mouse lines with the human phosphodiesterase 5A (PDE5A) 1 gene (6). As the mice were bred, it became evident that many males of one line were sterile and that the sterility arose from a defect in spermatogenesis (we named this mutant kisimo for a Japanese goddess of easy delivery). Because the sterility segregated with the hemizygous transgene and occurred in the absence of the detectable expression of the transgene, we concluded that the abnormal phenotype was due to mutagenesis by insertion of the transgene. In this study, to clone the junctions between the inserted transgene and adjoining cellular DNA, we used thermal asymmetric interlaced polymerase chain reaction (TAIL-PCR) (7) and defined a genomic locus important for spermatogenesis. The process of spermatogenesis in the mouse has been well characterized at the morphological level. The spermatogenic process can be subdivided into three main phases. Spermatogonia, the germinal stem cells, undergo mitosis to produce additional spermatogonia, a portion of which develop into primary spermatocytes. The spermatocytes enter meiosis and proceed through two cell divisions to give rise to haploid round spermatids. These, in turn, undergo a complex morphological transformation involving nuclear condensation and elongation resulting in the production of mature spermatozoa. However, at the molecular level, relatively little is known about the control of cellular differentiation and the architectural changes during spermatogenesis.In this study, we found that an insertion of foreign DNA results in abnormal male ...