COMP and osteogenesis imperfecta: Indeed, we have recently shown that, in both cell and mouse models of PSACH caused by COMP D469del, there is no evidence of a UPR (Suleman et al., 2012), whereas cell models of osteogenesis imperfecta caused by triple helical mutations in type I collagen do not show increased levels of BiP (Chessler and Byers, 1992) and are proposed to induce an ‘aggregated protein response’ (APR) (Makareeva et al., 2011).