Members of the RING finger family are implicated in a variety of functions such as signal transduction, transcriptional regulation and other developmental processes. Using degenerate oligonucleotide primers corresponding to the RING domain, we isolated a novel RING finger gene from the mouse testis cDNA library, which was about 1.8 kb and was termed Trif (testis-specific ring finger). This deduced protein contains an N-terminal RING-finger, a B-box, and a C-terminal B-30.2-like domain, which make the Trif protein a member of the RING finger-B-box-coil-coil family. Northern blot analysis of adult multiple tissues indicated that Trif is expressed predominantly in the testis. Further analysis detected Trif transcripts in the testis from day 20 of the postnatal stage. In situ hybridization indicated that Trif is expressed in the round spermatids of the seminiferous tubules. These expression data suggest that Trif may play an important role in the regulation of spermatogenesis.