Ossein is the organic extracellular matrix of bone, which is made of 95% collagen. It can be isolated by treating bones with hydrochloric acid, which dissolves the inorganic matrix (calcium phosphate and calcium carbonate). This substance is used in industry for the production of gelatin and bone glue. In the early 20th century, bones were found to consist of three types of proteins: ossein (collagens), osseomucoid (proteoglycans) and osseoalbuminoid (elastin). Advances in molecular biology rendered these terms obsolete.