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 1 or isoform 2 or any combination of isoform 1 and isoform 2. Found in collagen I-containing tissues: both isoform 1 and isoform 2 appear in amnion, chorion, skeletal muscle, small intestine, and in cell culture of dermal fibroblasts, keratinocytes and endothelial cells. Only isoform 2 is found in lung, placenta, kidney and a squamous cell carcinoma cell line. Isoform 1 is also present in the corneal epithelial Bowman's membrane (BM) and the interfibrillar matrix of the corneal stroma, but it is not detected in the limbal BM. The triple-helical tail is stabilized by disulfide bonds at each end. Hydroxylation on proline residues within the sequence motif, GXPG, is most likely to be 4-hydroxy as this fits the requirement for 4-hydroxylation in vertebrates. Isoform 1 O-glycosylation; glycosaminoglycan of chondroitin-sulfate type. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.