(A) Phosphorylation on serines (pSer), threonines (pThr), and tyrosines (pTyr) can be recognized by a subset of specialized PIDs. (B) Post-translational modifications on lysine by methylation (Me-Lys) mediate interactions with a large set of interaction modules whereas lysine acetylation (Ac-Lys) and arginine methylation (Me-Arg) are restricted to only Bromo and Tudor domains, respectively. (C) Modular PIDs that recognize short linear motifs. (X, represents any natural amino; ϕ, hydrophobic amino acids; -COOH, carboxyl terminus of amino acids)