A) Amino acid sequence of the catalytic loop region for Src (residues 384–391), Abl (residues 361–368), Csk, insulin receptor tyrosine kinase (IRK), and epidermal growth factor receptor kinase (EGFR). The D+2 Arg and D+4 Arg that are examined here are in bold. B) Schematic of the interaction of the catalytic loop residues corresponding to Asp363 and Arg367 in Csk with the substrate tyrosine, as observed in the crystal structure of inhibited IRK.