Type XII collagen interacts with type I collagen-containing fibrils, the COL1 domain could be associated with the surface of the fibrils, and the COL2 and NC3 domains may be localized in the perifibrillar matrix. Trimer of identical chains each containing 190 kDa of non-triple-helical sequences. The final tissue form of collagen XII may contain homotrimers of either isoform Long or isoform Short or any combination of isoform Long and isoform Short. Only isoform Long is a proteoglycan. Isoform Long has more restricted expression in embryonic tissue than isoform Short. Type XII collagen is present in tendons, ligaments, perichondrium, and periosteum, all dense connective tissues containing type I collagen. This sequence defines five distinct domains, two triple-helical domains (COL1 and COL2) and three non-triple-helical domains (NC1, NC2, and NC3). The triple-helical tail is stabilized by disulfide bonds at each end. Prolines at the third position of the tripeptide repeating unit (G-X-Y) are hydroxylated in some or all of the chains. O-glycosylated; glycosaminoglycan of chondroitin-sulfate type. Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.