Three-dimensional structure of a Nanobody^® (modeled on the VHH with pdb-code 1U0Q, Cambillau et al). The target molecule is recognized by the surface formed by three hypervariable regions located at the top of the molecule (in green, cyan and blue). Hallmarks, amino acid residues that further differentiate a camelid VHH from a human VH, can be located at different positions within the framework regions, and here are coloured red, pink and magenta.