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 or any combination of the various isoforms. Highest expression in tendons, perichondrium, skin, cornea, sclera, blood vessels, and periosteum. The long NC3 XIIA isoforms are predominant at early stages (ED7 and 11); at later stages of development (ED15 and 17) the short NC3 XIIB forms become the major forms. As the short NC3 forms become the major product, the long splice variant continues to be expressed in several tissues, even after birth. The long NC1 isoforms, XIIA-1 and XIIB-1, peak in 15-day old embryos and decrease in 17-day old ones. The expression of the short NC1 form XIIB-2 remains constant throughout late stages of embryonic development (ED15 and ED17). 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. O-glycosylation of isoform 2; glycosaminoglycan of chondroitin-sulfate type. Animals have fragile bones with a disorganized collagen fiber arrangement, decreased expression of bone matrix proteins, and decreased bone-forming activity associated with delayed terminal differentiation. They have also decreased grip strength, a delay in fiber-type transition, and a deficiency in passive force generation, while the muscle seemed more resistant to eccentric contraction-induced force drop, indicating a role for a matrix-based passive force-transducing elastic element in the generation of the weakness. Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.