Frequency of occurrence of amino acid residues after the final round. The frequency of occurrence of amino acid residues was calculated from 18 unique determined sequences of PAB variants after the final round. The ordinate indicates the percentage of the frequency of occurrence. The abscissa indicates the amino acid residue number. WT, wild-type residue: His, histidine residue: Others, non-wild-type and non–histidine residues. The number in the lower three rows indicate percentage.