Prp8 is a splicing factor of 220 kDa originally identified in yeast and is a component of the U5 small nuclear ribonucleoprotein particle. Mouse Prp8 cDNA was cloned and shown to share 62.6 and 68.2% sequence identity with the yeast homologue at the amino acid and nucleotide level, respectively, while it differs by only 3 amino acid residues from the human homologue. During mouse embryogenesis, Prp8 is expressed intensely at day 9.5 of gestation, and its expression decreases progressively during embryogenesis. In adult mice, Prp8 is expressed strongly in the testis and moderately in the ovary. In situ hybridization analysis revealed that Prp8 is preferentially expressed in the outer cell layer in the testis, probably in the spermatogonia and primary spermatocytes, and in granulosa cells in the ovary. In Caenorhabditis elegans, microinjection of a double stranded RNA corresponding to a portion of the Prp8 sequence results in the arrest of embryogenesis at the late-gastrulation stage. These results suggest that Prp8 plays an important role in reproduction and development. Prp8 was shown to bind to midkine (MK), a heparin-binding growth factor. Since Prp8 expression partially overlaps with the sites of action of MK, it is possible that binding to Prp8 is involved in part of MK signaling.