The crystals were flash frozen to a temperature of 100K before data collection in-house with a Rigaku RU-200 rotating-anode Xray generator. The data were processed and scaled with the XDS [57] . The crystal structure was solved using the molecular replacement method using the program Phaser [58] from the CCP4 suite of programs [59] . The search probe used to solve the structure was the structure of mouse MHC class I H2-D b minus the peptide (Protein Data Bank accession number 3CPL) [22] . The progress of refinement was monitored by the R free value with neither a sigma nor a low-resolution cutoff being applied to the data. The refinement protocol used includes several cycles of refinement with REFMAC [59] followed by manual model rebuilding with O program [60] . 'Translation, liberation and screw-rotation' displacement refinement was used to model anisotropic displacements of defined domains was used during the refinement process. The electron density around the NPN3A peptide was unambiguous, and all the side chains were built at full occupancy. Some mobile loops in the heavy chain of the H-2D b molecule (residues 191-201; 220-228 and 247-254) have been removed from the final model due to missing electronic density. Final refinement statistics are summarized in Table I , the coordinates of the H2Db-NP-N3A complex have been deposited with the Protein Data Bank under accession numbers 3FTG.