Serine protease that cleaves beta-casein/CSN2 as well as several extracellular matrix (ECM) proteoglycans such as decorin/DCN, biglycan/BGN and fibronectin/FN1. Inhibits signaling mediated by TGF-beta family proteins possibly indirectly by degradation of these ECM proteoglycans (By similarity). May act as a tumor suppressor. Negatively regulates, in vitro, trophoblast invasion during placental development and may be involved in the development of the placenta in vivo. May also have a role in ovarian development, granulosa cell differentiation and luteinization (PubMed:21321049, PubMed:22229724). Homotrimer (PubMed:26110759). Interacts with TGFB1; the interaction inhibits TGFB-mediated signaling. Interacts with BMP4; the interaction inhibits BMP4-mediated signaling. Interacts with TGFB2 and GDF5 (By similarity). Interacts with MYH9. Secretion increased during decidualization of endometrial stromal cells. Widely expressed, with highest levels in both adult and fetal heart, ovary, uterus placenta, and bladder. In the endometrium, expressed in epithelial glands and the stroma. Also present in leukocytes. Isoform 1 is predominant in heart and skeletal muscle, whereas isoform 2 is predominant in placenta and kidney. Down-regulated in ovarian and endometrial cancers (EC). Decrease of 3.2-fold in endometrial cancer. Belongs to the peptidase S1C family.