Background: Previously, we identified genes preferentially expressed during prophase I of meiosis and early folliculogenesis in the sheep ovary (1). We isolated a novel gene, named TOPAZ1 (Testis and Ovary-specific PAZ domain gene 1) present in male and female gonads. This gene is germ cell-specific and strongly expressed during male meiosis (2). Its nucleotide sequence is highly conserved in vertebrates and contained one PAZ and one CCH domains. Thus, Topaz1 could have an important role during germ cells development and gametogenesis. Aim: What is the consequence of Topaz1 gene deletion in mice? Material & Methods: We generated knock-in mouse and compared it to their wild-type counterpart.