Trivalent collagen cross-links can form as pyrroles. Hydroxylysyl-Pyrrole (HL-Pyrrole) is formed when Hydroxylysino-ketonorleucine (HLKNL) reacts with hydroxylysino-norleucine (deH-HLNL) (Eyre et al. 2008). The mechanism of pyrrole cross-links has been revised to a structure based on 3-hydroxypyrrole, rather than a pyrrole lacking a hydroxyl on the ring as depicted earlier (Bailey et al. 1998).