Integrins are membrane receptors that mediate cell-cell or cell-matrix adhesion. All integrins are transmembrane heterodimers composed of α and β subunits and are connected to the cytoskeleton. At least 20 integrins, formed from combinations of 12 α and 9 β subunits, have been reported. Many of these have been implicated as transducers of molecular signals. The β1 subgroup of this receptor family comprises at least six different dimer combinations. Among these combinations, α2β1 is associated with type I collagen and laminin binding and regulation, α3β1 is a receptor for laminin and epiligrin, and α5β1 is a fibronectin receptor. β1 integrins play an important role in several aspects of epidermal differentiation and morphogenesis. Expression of the β1 subunit is regulated by growth factors such as TGF-β1. Integrin activation, which enhances binding of T cells to endothelium, is regulated in part by phosphatidylinositol 3-kinase. Focal adhesion kinase (FAK) and paxillin have been reported to independently bind the C-terminal, cytoplasmic domain of the β1 subunit. FAK is reported to be enzymatically activated upon engagement of integrins with their ligands and paxillin is phosphorylated on tyrosine upon activation of FAK.