Protein tyrosine kinase 2 (PTK2 a.k.a. Focal Adhesion Kinase (FAK); EC 2.7.10.2) is a cytoplasmic non-receptor tyrosine kinase which is found concentrated in the focal adhesions that form between cells growing in the presence of extracellular matrix constituents. It was originally identified as a substrate for viral Src and as a highly tyrosine-phosphorylated protein that localized to cell adhesion sites known as focal contacts. FAK has been shown to have a key role in both normal and tumor cell migration downstream of growth factor- and integrin-receptors. It is the formation of a FAK–Src signaling complex that is an initial and important event required for maximal FAK activation and cell migration. Activation is involved in modulating \'corrective\' cell responses to environmental stimuli, which is provoked by signal-mediated effects on actin polymerization, the assembly or disassembly of focal contacts, and the regulation of protease activation or secretion.