(A) Evolutionary similarities of protein structures Two decades have passed from the time that the three dimensional structure of the first globular protein, sperm whale myoglobin, was decoded (Kendrew et al. 1960). Its structure, which now looks so simple and habitual, then seemed to be unusually complicated. The decoding of the subsequent proteins, lysozyme (Blake et al. 1965), ribonuclease (Kartha, Bello & Harker, 1967), chymotrypsin (Matthews et al. 1967), carboxypeptidase (Lipscomb et al. 1969) redoubled the feeling of amazement and even of some confusion before the extremely complicated, intricate and, above all, absolutely unlike protein structures. Some consolation against this background was the evident and far-reaching similarity between the three-dimensional structures of myoglobin and hemoglobin subunits (Perutz, Kendrew & Watson, 1965) and an analogous similarity between the structures of chymotrypsin and other serine proteases, elastase (Shotton & Watson, 1970) and trypsin (Stroud, Kay & Dickerson, 1972). However this similarity was easily explained by the far-reaching homology between the primary structures of myoglobin and hemoglobin and between the primary structures of serine proteases.