Procollagen type I carboxy-terminal propeptide (PICP), derived from type I procollagen, has been identified as an indicator of type I collagen synthesis in bone matrix formation and skin recovery. PICP is a heterotrimeric glycoprotein consisting of two α1 chains (PICPα1) and one α2 chain (PICPα2). The cross-linked C-terminal telopeptide of type I collagen, or CTX-I, is a sensitive but also rather variable marker of bone resorption, and several assays for the cross-linked telopeptide are commercially available.