Our laboratory have pointed out and characterized a new gene which was called TOPAZ1 (Testis and Ovary-specific PAZ domain gene 1) (Baillet el al, 2011). Because this gene is highly conserved in Vertebrates and is specifically expressed in germ cells, the role of TOPAZ1 has been studied during gametogenesis. For this, the mouse Topaz1 gene has been disrupted via a knock- in model. Whereas the fertility of invalidated female mice is not disturbed, homozygous Topaz1-/- male are sterile. They have a reduced testis size compared to wild type testis and histological analysis highlighted an absence of spermatids (round and elongated) and spermatozoa in Topaz1-/- testis. These analyses also revealed that the perturbation of spermatogenesis takes place between 15 and 20 dpp. The lumen of their epididym is also completely devoid of spermatozoa. Thus, meiosis seems to stop before haploid germ cells formation. Moreover, we showed that neither retrotransposon repression in germ cells nor chromosome pairing during prophase I of meiosis are disturbed in Topaz1-/- testis (Luangpraseuth-Prosper et al., in preparation). Transcriptomic analysis have been realized from two developmental stages (15 and 20 dpp) of controls and Topaz1-/- testis and showed gene expression variations. The validation of the differential expressed genes by quantitative PCR and the characterization of the biological pathways disturbed in absence of Topaz1 are on the way in the laboratory. All of these analyzes should point out a better understanding ofTopaz1 function in mammals spermatogenesis.