FAK, (Focal Adhesion Kinase), also known as PTK2(protein tyrosine kinase 2), is a protein that, in humans, is encoded by the PTK2 gene. PTK2 is a focal adhesion-associated protein kinase involved in cellular adhesion (how cells stick to each other and their surroundings) and spreading processes (how cells move around). It has been shown that when FAK was blocked, breast cancer cells became less metastastic due to decreased mobility. This gene encodes a cytosolic protein tyrosine kinase that is found concentrated in the focal adhesions that form among cells attaching to extracellular matrix constituents. The encoded protein is a member of the FAK subfamily of protein tyrosine kinases that included PYK2 but lacks significant sequence similarity to kinases from other subfamilies. With the exception of certain types of blood cells, most cells express FAK. FAK tyrosine kinase activity can be activated, which plays a key important early step in cell migration. FAK activity elicits intracellular signal transduction pathways that promote the turn-over of cell contacts with the extracellular matrix, promoting cell migration. FAK is required during development, with loss of FAK resulting in lethality. It seems to be a paradox that FAK is not absolutely required for cell migration, and may play other roles in the cell, including the regulation of the tumor suppressor p53. At least four transcript variants encoding four different isoforms have been found for this gene, but the full-length natures of only two of them have been determined. FAK is a protein of 125 kD recruited as a participant in focal adhesion dynamics between cells, and has a role in motility and cell survival. FAK is a highly conserved, non-receptor tyrosine kinase originally identified as a substrate for the oncogene protein tyrosine kinase v-src. This cytosolic kinase has been implicated in diverse cellular roles including cell locomotion, mitogen response and cell survival. FAK is typically located at structures known as focal adhesions, these are multi-protein structures that link the extracellular matrix (ECM) to the cytoplasmic cytoskeleton. Additional components of focal adhesions include actin, filamin, vinculin, talin, paxillin and tensin.