Structure of the affibody molecule. (A) Three-dimensional structures, showing a full-length IgG antibody (PDB ID 1HZH), a Fab antibody fragment (PDB ID 6MH2), an scFv antibody fragment (PDB ID 1X9Q), and an affibody molecule (PDB ID 2B88). The proteins are shown in scale, and the molecular weights are indicated below the corresponding protein name. (B) Three-dimensional structure of an affibody molecule, with the 13 randomized positions on helices 1 and 2 indicated. (C) Amino acid sequence in single-letter code of the protein A–derived Z_wt domain, with the three α-helices boxed and the randomized positions indicated.