Collagens are the major fibril-forming proteins in the ECM and they consist of a basic triple-helical conformation. The triple helix increases molecular stability and provides resistance to tensile stress. Although many types of collagen exist, a consistent pattern can be observed for amino acid sequences of all collagens; each chain contains enriched triplet repeats consisting of the sequence Xaa-Pro-Gly, where Xaa is any amino acid, Pro is proline, and Gly is glycine. Prolines within these domains become hydroxylated under the action of prolyl-hydroxylases.78 Hydroxyprolines provide the substrate for the formation of hydrogen bonds between the adjacent collagen alpha chains. Prolyl-4-hydroxylases and prolyl-3-hydroxylase catalyse the hydroxylation of specific proline residues. The former enzyme reacts on proline with the minimum sequence Xaa-Pro-Gly and the latter appears to require a Pro-4Hyp-Gly (Hyp is hydroxyproline) sequence.78,79 Hydroxylation is a stable, non-reversible PTM that adds +15.99 Da (i.e. an oxygen atom) to proline. Xaa-Pro-Gly domains are rare in ECM proteins other than collagens, and this confers specificity to the acquisition of this PTM.