Involved in embryo hatching and implantation. Inhibited by benzamidine, (4-amidino-phenyl)-methane-sulfonyl (APMSF), N-p-tosyl-L-lysine chloromethylketone (TLCK), gabexate, mesylate, BABIM and trypsin soybean inhibitor (TSI). 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 throughout the preimplantation period, during blastocyst hatching and embryo outgrowth. Found in uterus especially in glandular epithelium. Highly expressed from 6.5 dpc in embryo plus deciduum. Faintly detectable at 11.5 dpc and 13.5 dpc in placenta. Not detected at 8.5 dpc, 11.5 dpc and 13.5 dpc in embryo proper. Expressed at 4.0 dpc in uterus. By progesterone. Belongs to the peptidase S1 family.