Donald F. Steiner is known for his discoveries on proinsulin, the precursor of insulin which is a hormone that helps the body control the use of sugar. He found that insulin is synthesized from a larger precursor protein in the beta cells of the pancreas and this led the way to elucidation of how the islet cells function, and how peptide hormones are synthesized and metabolized. He and his colleagues discovered some of the enzymes that convert proinsulin into insulin, and also devised methods for measuring insulin and its precursors in human serum.