Hydroxylysine glycosides are specific to collagen. Collagen glycosylation takes place in the endoplasmic reticulum before triple-helix formation. Either galactose or glucose-galactose are attached to approximately one third of hydroxylysine residues by specific transferases, beta(1-O)galactosyl- and alpha(1-2)glucosyltransferase, forming galactosyl hydroxylysine (Gal-Hyl) and glucosyl-galactosyl hydroxylysine (Glu-Gal-Hyl) respectively. The genes GLT25D1 and GLT25D2 encode galactosyltransferases that are active with various types of collagen and the serum mannose-binding lectin MBL, which also contains a collagen domain. GLT25D1 gene is constitutively expressed in human tissues, whereas the GLT25D2 gene was found to be expressed only at low levels in the nervous system. These galactosyltransferases convert 5-hydroxylysine to 5-galactosyl hydroxylysine (Gal-Hyl). The extent of hydroxylysine galactosylation is variable between collagen types and locations; it is particularly common in bone type I collagen (Al-Dehaimi et al. 1999). Although the fraction of hydroxylysine residues that are glycosylated does not differ between skin and bone (the major sources of type I collagen) the pattern of hydroxylysine glycosylation is different. Glu-Gal-Hyl predominates in skin, where the Glu-Gal-Hyl/Gal-Hyl ratio is approximately 2 (Pinnell et al. 1971), whereas Gal-Hyl predominates in bone, where the Glu-Gal-Hyl/Gal-Hyl ratio is 0.47 (Krane et al. 1977).