BIR domain and its inhibitory SMAC peptide derived from x-ray crystallography. (A) The x-ray structure of the third BIR domain of XIAP (ribbon drawing) in complex with the tetra-peptide AVPI derived from SMAC (ball-and-stick). (B) The surface of the third BIR domain of XIAP is shown and color coded according to cavity depth: blue, shallow; yellow, deep. The AVPI tetra-peptide is shown in ball-and-stick representation. The figures were generated with MOLMOL (A) and MOLCAD (B) as implemented in Sybyl (Tripos).