A: Ubc9 possesses a specific insertion of two amino acids between the catalytic cysteine and tryptophane W103. Sequences of human UbcH5B (GI: 1145689), human E2-25k (GI: 1381164), human UbcH6 (GI: 1064914), human Ubc12 (GI:4507791), human Ubc9 (GI:4507785), and S. cerevisae Ubc9 (GI:1431070) were aligned using Clustalw with default parameters. The catalytic cysteine is shown in bold. Identical residues are indicated by an asterisk under the aligned sequences. The model of Ubc9 (PDB: 1A3S) was generated using Deepview and rendered using POV-Ray. 