The ROCK family of Rho-associated serine-threonine protein kinases is known to play an important role in Rho-mediated cell adhesion and smooth muscle contraction. Y-27632 2HClis a potent, ATP-competitive inhibitor of ROCKs including p160ROCK (Ki = 140 nM) and ROCK2 (IC50 = 800 nM). It also inhibits PRK2 with an IC50 value of 600 nM. One-hour treatment with 10 μMY-27632 2HCl blocks apoptosis of dissociated cultured human embryonic stem cells, increasing cloning efficiency by 25% and sustaining survival up to 30 passages.