HLA class II polymorphism and three-dimensional structure. [PDB:1A6A] is used as a representative structure in this figure. (A) Ribbon diagram of the binding groove. Chain α, grey; chain β, red. Each chain consists of two domains (α1, α2 and β1, β2, respectively). Only the ectodomains α1 and β1 shape the binding groove. (B), as (a), but showing the entire molecule. (C) Three-dimensional representation of the sequence variability in the binding groove, highlighting the location of SDRs in the β1 domain. Variability is represented in a two-colour scale from blue to red, where blue indicates non-variable positions and red indicates variable positions.