A 20-kDa sperm membrane protein cDNA, designated as RSD-1, was isolated by epitope selection from a rat testis lambda gtll expression library. RSD-1 was used as a probe to screen a human testis lambda ZAPII cDNA expression library. A cDNA designated as BS-63 was isolated and found to consist of 1933 bp with an open reading frame of 1824 bp and assigned the accession number U64675 by GenBank. The deduced polypeptide consisted of 608 amino acid residues containing XFXFG or FG motifs that are characteristic of nuclear pore complex (NPC) proteins and act as potential binding sites for Ran. The N-terminal region has high homology with RanBP2/Nup358, a nucleoporin component, showing that BS-63 is a member of the NPC family. Northern blot analysis of mRNAs prepared from various human tissues shows that BS-63 is transcribed in two forms: 6.0 and 8.5 kb. The 8.5-kb transcript was present in low amounts in several somatic tissues, whereas the 6.0-kb transcript is expressed only in testis. In situ hybridization analysis of human testis sections showed that BS-63 mRNA is expressed only in germ cells at all stages of spermatogenesis. Sertoli cells did not transcribe the gene.