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The pregnancy-associated glycoproteins (PAGs) are placental antigens that were initially characterized as pregnancy markers in the maternal circulation of domestic ruminant species. They are members of the aspartic proteinase gene family, having greatest sequence identity with pepsinogens. However, some are not capable of functioning as enzymes. The PAGs are associated with a large gene family within the Artiodactyla order (cattle, camels, pigs). So far, no members of this family have been characterized in species outside this order. This report describes the cloning and initial characterization of a PAG-like protein (equine PAG or ePAG) expressed in the placenta of the horse and zebra (order Perrisodactyla). Equine PAG is a proteinase capable of degrading 14C-hemoglobin and catalyzing the removal of its own pro-peptide. The ePAG mRNA is restricted to the chorion both prior to implantation and in the term placenta. Equine PAG is secreted from cultured placental tissue as both a processed (mature) and unprocessed (zymogen) form. Equine PAG shares similar identity with the PAGs and pepsinogens and probably arose from a pepsinogen-like precursor that gained the ability to be expressed in the placenta. The promoter of the ePAG gene shares sequence identity with the promoter from a bovine PAG gene but not with promoters of other aspartic proteinases. Therefore, we hypothesize that ePAG is a remnant of the pepsinogen-like progenitor gene that was expanded within the Artiodactyla to create the large and highly diverse PAG family.