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 (PubMed:15206957). 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 (By similarity). Homotrimer (By similarity). Interacts with MYH9 (By similarity). Interacts with TGFB1; the interaction inhibits TGFB-mediated signaling. Interacts with BMP4; the interaction inhibits BMP4-mediated signaling. Interacts with TGFB2 and GDF5. Secretion increased during decidualization of endometrial stromal cells. Highest level of isoform 1 in maternal part of the placenta, moderate level in heart, testis and ovary, low level in muscle and lung. High expression found in granulosa cells of the ovary. Expressed in bone matrix, particularly in articular chondrocytes. Very low level of isoform 2 expressed in placenta. Expressed in the bone matrix, particularly in articular chondrocytes. First expressed at 9.5 dpc. Levels then increase until 14.5 dpc after which they remain high until newborn. First detected in the eye at 10.5 dpc and then expressed in tissues associated with skeletal tissue. At 12.5 dpc expressed in the vertebral rudiments in the tail region and, in the developing eye, in lens epithelium. At adulthood, expression found in the ganglion cell layer and the inner nuclear layer of the retina. In the developing heart at 16.5 dpc, expressed in the endocardial cushion. In the trachea at this stage, in outer layers and in the aorta, in adventitia. Up-regulated during early pregnancy coinciding with placentation. Also up-regulated in joint cartilage affected by arthritis. Belongs to the peptidase S1C family.