"In 1949, application of methods of physical chemistry directly to the study of a protein produced by a mutated gene led Pauling, Itano, Singer and Wells to identify the specific change in the protein brought about by the gene. The discovery of the first of the abnormal human hemoglobins which they described as causing a 'molecular disease' -- sickle cell anemia -- was followed the identification of a large number of other proteins, each of which owed its difference from normal structure to a mutated gene. Ingram then showed that the change due to the mutation, in the case of each of two abnormal hemoglobins, was confined to a single amino acid residue at one point in one of the polypeptide chains composing the globin. There could be no doubt that genes controlled protein structure by specifying the sequence of amino acid residues in the polypeptide chains. The assumed basic functional correspondence was then altered from 'one gene-one enzyme' to 'one gene-one polypeptide.'"

L. C. Dunn. "Old and New in Genetics." Bulletin of the New York Academy of Medicine, 40(5): 325-333, 329. May 1964.