Identification of genes specifically expressed in adult and fetal testes is important in furthering our understanding of testis development and function. In this study, a novel human specific transcript, designated NYD-SP10, was identified by hybridization of adult and fetal human testis probes with a human cDNA microarray. NYD-SP10 (GenBank Accession no. AF332192) was expressed at a higher level in adult testes than in fetal testes. Bioinformatic analysis illustrated NYD-SP10 is a splice variant of regulatory factor X4, while it is lack of RFX DNA-binding domain which binds to the X box of MHC class II genes. Most telling of all, NYD-SP10 uniquely possesses a tyrosine kinase phosphorylation site. Expression profiles demonstrated NYD-SP10 is specifically and strongly expressed in human adult testes and ejaculated spermatozoa. Taken together, these results suggest that NYD-SP10 is a novel testis-specific gene and it is possibly a novel substrate for tyrosine kinase and involved in the function of human spermatozoa, especially during capacitation.