Protein expression, purification, crystallisation and structure determination H2-D b and b2-microglobulin molecules were expressed in Escherichia Coli as inclusion bodies, refolded with the NP-N3A (ASAENMETM) peptide and purified as previously described [55, 56] . The H2D b NP-N3A complex crystals were obtained at 3mg/ml by the hanging-drop vapour diffusion technique at 20uC. Crystals were grown with a reservoir containing 0.1 M sodium citrate at pH 5.7, 28% PEG 3350 (w/v), 0.2 M lithium sulphate. The crystals belong to space group I222 and the unit cell dimensions were consistent with one molecule per asymmetric units (Table S1 ).