Zp1 encodes one of the three major glycoproteins of the zona pellucida, an extracellular matrix that surrounds growing oocytes, ovulated eggs, and preimplantation embryos. The mouse gene is composed of 12 exons ranging in size from 82 to 364 base pairs and spans 6.5 kilobase pairs on chromosome 19 (2.13 +/- 1.5 centimorgans distal to D19Bir1). The Zp1 exon map is similar to ZPB, a human orthologue, and an E-box (CANNTG), implicated in oocyte-specific gene expression of mouse Zp2 and Zp3, is similarly located upstream of the transcription start site. The single copy Zp1 gene encodes a 623-amino acid protein, the carboxyl-terminal half of which is significantly similar to a corresponding region of mouse ZP2. The conservation of this same region in a fish egg envelope protein suggests that not only has this protein domain been duplicated in mammals but that it has been conserved and used as an egg envelope protein in species that diverged 650 million years ago.