nd simultaniously but weakly to large numbers of matrix molecules and still have the uppertunity to explore their environment without losing all attachment to it by building or breaking down focal c!
             ontacts. If the receptors were to bind strongly to their ligands, cells would probably be irreversably bound to the matrix, depriving it from motility. This problem does not arise when attachment depends on multiple weak adhesions. Integrins are composed of long extracellular domains which adhere to their ligands, and short cytoplasmic domains that link the receptors to the cytoskeleton of the cell. There is however one exception to this rule, which is the beta-4 subunit. Beta-4 contains a very large cytoplasmic domain of some 1000 amino acids, while the other integrins only have cytoplasmic domains of up to 60 amino acids. The structure between the alpha
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