superfamily of cell surface proteins that are involved in binding to extracellular matrix components in some cases. most are heterodimeric with a _ subunit of 95 kd that is conserved through the superfamily and a more variable _ subunit of 150-170 kd. The first examples described were fibronectin and vitronectin receptors of fibroblasts, which bind to an RGD (arg gly Asp) sequence in the ligand protein, though the context of the RGD seems important and there is also a divalent cation dependence. Subsequently the platelet IIb/IIIa surface glycoprotein (fibronectin and fibrinogen receptor) and the lfa 1 class of leucocyte surface protein were recognised as integrins, together with the VLA surface protein. The requirement for the RGD sequence in the ligand does not seem to be invariable.