The principal receptors on animal cells for binding most extracellular matrix proteins—including collagens, fibronectin, and laminins—are the integrins. Integrins, like other cell adhesion molecules, differ from cell-surface receptors for hormones and for other extracellular soluble signal molecules in that they usually bind their ligand with lower affinity and are usually present at about tenfold to a hundredfold higher concentration on the cell surface. If the binding were too tight, cells would presumably become irreversibly glued to the matrix and would be unable to move—a problem that does not arise if attachment depends on large numbers of weak adhesions. This is an example of the “Velcro principle” mentioned earlier. Like other transmembrane cell adhesion proteins, however, integrins do more than just attach a cell to its surroundings. They also activate intracellular signaling pathways that communicate to the cell the character of the extracellular matrix that is bound.