Sequence alignments and structural superpositions of caspases. A, Polypeptide sequence alignments of human caspases were performed using the Clustal W program  and then adjusted manually. Amino acid residues are numbered to the right of each sequence. Active site residues are highlighted in red, and S1 (■), S2 (□), S3 (●), and S4 (○) subsites on the active sites are indicated. B, Structural superpositions of Cα atoms of caspases-3 (blue), -7 (pink), -8 (green), and -9 (orange) are presented in line representation, and active site residues on the three-dimensional structures of caspase-3 are red in space-fill representation. The superpositions were performed using the McLaghlan algorithm  as implemented in the ProFit program .