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The three-dimensional structure of the class II histocompatibility glycoprotein HLA-DR¹ from human B-cell membranes has been determined by X-ray crystallography and is similar to that of class I HLA. Peptides are bound in an extended conformation that projects from both ends of an 'open-ended' antigen-binding groove. A prominent non-polar pocket into which an 'anchoring' peptide side chain fits is near one end of the binding groove. A dimer of the class II αβ heterodimers is seen in the crystal forms of HLA-DR1, suggesting class II HLA dimerization as a mechanism for initiating the cytoplasmic signalling events in T-cell activation.