Certain fibril-associated collagens with interrupted triple helices (FACITs) associate with the surface of collagen fibrils, where they may serve to limit fibril fusion and thereby regulate fibril diameter (Gordon & Hahn 2010, Ricard-Blum et al. 2011). Collagen IX cross-linked to the surface of collagen type II fibrils is thought to both regulate fibril diameter and stabilize interfibrillar connections (Eyre et al. 2004). An alternative model suggests that collagen II and XI form a biological alloy (Blaschke et al. 2000).