Stiffness and elasticity also guide cell migration, this process is called durotaxis. The term was coined by Lo CM and colleagues when they discovered the tendency of single cells to migrate up rigidity gradients (towards more stiff substrates) and has been extensively studied since. The molecular mechanisms behind durotaxis are thought to exist primarily in the focal adhesion, a large protein complex that acts as the primary site of contact between the cell and the ECM. This complex contains many proteins that are essential to durotaxis including structural anchoring proteins (integrins) and signaling proteins (adhesion kinase (FAK), talin, vinculin, paxillin, α-actinin, GTPases etc.) which cause changes in cell shape and actomyosin contractility. These changes are thought to cause cytoskeletal rearrangements in order to facilitate directional migration.