Structural basis for PP1 selectivity. The 3D structure of CDK2 (PDB 2A4L, wire rendering with grey carbons) was superposed onto the structure of HCK (hidden) in complex with PP1 (PDB 1QCF, ball and stick rendering with green carbons). PP1 makes a number of clashes (orange lines) with Phe 80 (ball and stick rendering with grey carbons) of CDK2 whereas Thr 338 (stick rendering with green carbons) of HCK accommodates the compound. By mutating the threonine to a bulkier residue, Liu et el demonstrated that this position was an important specificity determinant for PP1. We therefore conclude that S-Filter correctly predicted Thr 338 as a specificity determinant.