A novel testis-specific gene termed mtIQ1 (GenBank Accession No. DQ153246) was identified by digital differential display. Sequence analysis revealed that mtIQ1 protein is a new member of calmodulin (CaM) binding protein families with conserved Ile and Gln residues (IQ motif). RT-PCR and Northern blot analysis revealed that a 0.9 kb mtIQ1 transcript was only expressed in adult mouse testis and not expressed in nine other tissues. The expression of mtIQ1 mRNA was developmentally upregulated in the testes during sexual maturation and was, conversely, downregulated by experimental cryptorchidism in vivo. The green fluorescence produced by pEGFP-C3/mtIQ1 was detected mainly in the nucleus of GC-1 cells 24 h post-transfection. Results of in situ hybridization assay confirmed that mtIQ1 was expressed in seminiferous tubules, more precisely in spermatocytes. The testis-specific and time-dependent expression pattern of mtIQ1 in postnatal mouse testes suggested that it might be involved in the regulation of spermatogenesis and sperm maturation.