Integrin control of proliferation. Synergistic activation of Ras and Rho GTPases by integrins and growth factors is necessary for cell cycle progression. Activation of Rac1, Cdc42, and RhoA can facilitate activation of Erk via Ras. Erk, as well as JNK and ILK, can induce expression of cyclin D1, thus promoting cell cycle progression. In addition, integrins can promote proliferation by inhibiting cell cycle inhibitors like p21^cip and p27^kip.