FAK is a widely expressed non-receptor protein tyrosine kinase discovered as a substrate for Src and as a key element of integrin signaling. Regulation of FAK includes phosphorylation at multiple tyrosine and serine residues. Increased FAK tyrosine phophorylation occurs upon integrin engagement with fibronectin. Phosphorylation of tyrosine generally is associated with positive regulation and growth promotion; however, dephosphorylation at these sites occurs as cells begin to enter the M-phase of the cell cycle. In contrast, serine phosphorylation either remains high or is increased as cells enter mitosis. FAK plays a central role in cell spreading, differentiation, migration, cell death and acceleration of the G1 to S phase transition of the cell cycle. FAK binds multiple signaling proteins including p130Cas, Graf, Grb2, Src family SH2 domains, and the p85 subunit of PI3-kinase. The epitope is localized in the C-terminus (amino acid residues 853-1052).