Beginning from a mouse EST (GenBank Accession No: BE644537) which was significantly changed in cryptorchidism and represented a novel gene, GeneScan program was performed and a predicted mouse novel gene full-length cDNA sequence containing the BE644537 sequence was attained. Gene-specific primers were designed for PCR in mouse testis cDNA library. The sequencing result of the PCR product showed that we obtain a new gene mTSARG3 (GenBank Accession No: AF419292) whose full cDNA length is 1328 bp containing 8 exons and 7 introns. The predicted open reading frame encodes a protein of 316 amino acid residues. The protein encoded by the new gene is a new member of HSP40 protein family because the sequence contains the highly conserved J domain which is present in all DnaJ-like proteins and is considered to have a critical role in DnaJ-DnaK protein-protein interactions. mTSARG3 protein displays a 46% identity in a 336-amino-acid overlap with DJB4-MOUSE protein. The result of RT-PCR analysis and Northern blot showed that mTSARG3 is specifically expressed in mouse testis. Southern blot showed that there were no loss and rearrangement in cryptorchid. Based upon all these observations, it is considered that the function of the new gene is related to inhibit mouse testis spermatogenesis apoptosis.