The peptide-binding cleft of the HLA class II molecule. The peptide is bound by virtue of the physicochemical interaction between the cleft and the anchor residues of the peptide. The TCR contacting residues along with residues on the α1 and β1 domains (see Figure 1) determines T-cell recognition and would represent a NPPE. The molecular model was generated and kindly provided by Peter Hjelmström (for additional models, see http://depts.washington.edu/rhwlab/dq/intro.html).