[Part (a) data from L. Gonzalez, Jr., D. N. Woolfson, and T. Alber, 1996,
Nat. Struct. Biol.
3:101
1–1018, PDB IDs 1zik and 2tma. Part (b) data from R. Chattopadhyaya et al., 1992, J. Mol. Biol. 228:1177–1192, PDB ID 1cll. Part (c) data from S. A. Wolfe, R. A. Grant, and C. O. Pabo, 2003, Biochemistry 42:13401–13409, PDB ID 1llm.]