The phosphopeptide EPQ(p)YEEIPIYL is derived from the hamster polyoma middle-T antigen. It binds with high affinity and specificity to the Src homology-2 (SH2) domains of the protein tyrosine kinases p56 lck and p60 src. The side chains of residues pY (phosphotyrosine) and Ile at position pY+3 were shown to occupy deep binding pockets of either SH2 domain surface.