Involved in embryo hatching and implantation. Homooligomer, heterodimer and heterotetramer. Able to form homo- and hetero- tetrameric structures. Heterotetramer is far more stable than the homotetramer. Secretion into the glandular and uterine lumen may occur as a consequence of progesterone-induced epithelial differentiation. Expressed in embryos and placenta. Found in uterus especially in glandular epithelium during zona lysis and implantation. Highly expressed from 6.5 dpc plus deciduum. Also expressed, but to a lower extent, in placenta from 11.5 dpc and 13.5 dpc. Not expressed at 8.5 dpc and 11.5 dpc in embryo proper. Expressed at 4.0 dpc in uterus. Up-regulated in uterine endometrial glands following the initiation of embryo implantation. By progesterone. Belongs to the peptidase S1 family.