Domains. Proteins fold into one or more domains, depending on their size. (a) One domain: sperm whale myoglobin. (b) Two domains: human γ crystallin. (c) Three domains: Aquifex aeolicus DnaA. (d) Four domains: E. coli Pol I Klenow fragment; the DNA polymerase activity is contained in the domains numbered 1, 2, and 3, and the exonuclease activity is in the fourth domain.