We identified a novel gene, termed GSE (gonad-specific expression gene). Nucleotide sequence analysis of GSE cDNA revealed that the open reading frame of 745-bp encodes a protein of 247 amino acids with a predicted molecular mass of 27.6 kDa. The deduced amino acid sequence indicated that GSE protein might be a soluble protein in the cytoplasm without a signal peptide. Northern blot analysis showed that this gene was abundantly expressed in mouse testis and slightly expressed in the mouse ovary. RT-PCR analyses indicated that the GSE mRNA in the testis was first detected at Day 14 postpartum, when spermatocytes at mid-pachytene are likely to appear. In situ hybridization confirmed its expression at this stage of spermatogenesis. On the other hand, the GSE mRNA in the ovary was already present at birth, when germ cells are in meiosis. These observations suggest that GSE may be associated with meiosis during gametogenesis.