The cDNA for an RNA that is expressed predominantly in mouse spermatogenic cells was cloned and characterized. It was found to encode novel zinc finger protein. We first generated a cDNA fragment from mouse osteoblastic cells by the differential display method. To our surprise, Northern blot analysis revealed that the corresponding transcript was expressed at high levels in the testis rather than in osteoblastic cells. Therefore, using this fragment as a probe, we isolated the full-length cDNA (3340 bp) from a mouse testis cDNA library. Analysis of the open reading frame of the cDNA indicated that the encoded protein was a polypeptide of 942 amino acids residues that included three distinct domains, namely, a zinc finger domain of the Cys(2)-His(2) type, four basic amino acid-rich domains, and a myosin II-homology domain. In situ hybridization indicated that the transcript was present in seminiferous tubules of adult mice. Elevated expression of the transcript during testicular development in mice was restricted to spermatocytes at the pachytene stage of meiotic prophase and to round and elongated spermatids, as indicated by Northern blot analysis and RT-PCR. Our results suggest that this novel zinc finger protein might act as a transcriptional regulator during spermatogenesis and, in particular, during meiotic division.