However, a second puzzling feature of the crystal structure remains unexplained after this revision. The truncated N-terminus of the receptor, which is highly disordered and hence unresolved in other opioid receptor structures, unexpectedly intrudes into the binding pocket . The third residue, His54, clashes with BU72. The overlapping atoms also contact a pocket of strong, unexplained electron density (Figure ). The atom responsible for this density could not be identified; experiments testing for an alternative ligand structure or a coordinated heavy metal were unsuccessful . The atom was ultimately omitted from the model altogether. The revised model with 1b