The MEROPS website has two displays to show peptidase specificity. Both use the data from natural and synthetic substrates, but show only naturally occurring amino acids. The first display is a logo which uses the WebLogo software (27) . Residues P4-P4 0 from all the substrates for a peptidase are treated as an alignment. The observed frequency for each amino acid in each position is calculated as a bit score, the maximum possible being 4.32 bits. An amino acid is shown in the logo (in single-letter code) if the bit score exceeds 0.1. The logo is also shown as a text string, where if a single amino acid predominates at one position (i.e. the bit score exceeds 0.4) the letter is shown in uppercase, and if more than one amino acid predominates in any position a letter is shown in uppercase when the bit score exceeds 0.7 and in lower case if the bit score is between 0.1 and 0.7.