p125FAK tyrosine phosphorylation is a primary response to integrin stimulation. The first evidence that integrin occupancy by ligands induces tyrosine phosphorylation came from studies on the fibrinogen receptor αllbβ3 in platelets (1 ). Further work showed that both in normal and transformed cells several different integrin receptors can trigger tyrosine phosphorylation of intracellular proteins following interactions with matrix proteins or integrin receptor clustering by specific antibodies (2 ).