The extracellular matrix (ECM) is a complex meshwork of non-cellular material that surrounds cells in all tissues and organs. Correct assembly of ECM proteins is known to involve posttranslational processing in the endoplasmic reticulum (ER), but the exact mechanisms are only partially known. Recently it has been shown that autosomal-recessive forms of osteogenesis imperfecta may be due to deficiencies of ER chaperones.