The second display is a frequency matrix, which is an 8 Â 20 matrix with residues P4-P4 0 along the x-axis and all amino acids along the y-axis. The amino acids are ordered so that those with similar properties are adjacent. The order is Gly, Pro, Ala, Val, Leu, Ile, Met, Phe, Tyr, Trp, Ser, Thr, Cys, Asn, Gln, Asp, Glu, Lys, Arg and His. Preference is calculated in terms of the percentage of substrates with each amino acid in each position, and a different shade of green is used for each tenth percentile interval. The number of times a residue occurs at each position is shown.