 Type 1 encodes FKBP65, an endoplasmic reticulum associated peptidyl-prolyl cis/trans isomerase (PPIase) that functions as a chaperone in collagen biosynthesis.Osteoblasts deficient in FKBP65 have a buildup of procollagen aggregates in the endoplasmic reticulum which reduces their ability to form bone.Furthermore, Bruck syndrome type 1 patients have under-hydroxylated lysine residues in the collagen telopeptide and as a result show diminished hydroxylysylpyridinoline cross-links.