Selective inhibition of extracellular sign at-regulated kinase (ERK) represents a potential approach for the treatment of cancer and other diseases; however, no selective inhibitors are currently available. Here, we describe an ERK-selective inhibitor, FR180204, and determine the structural basis of its selectivity. FR180204 inhibited the kinase activity of ERK1 and ERK2, with K-i values 0.31 and 0.14 mu M, respectively. Lineweaver-Burk analysis of the binding interaction revealed that FR180204 acted as competitive inhibitor of ATP. In mink lung epithelial. Mv1Lu cells, FR180204 inhibited TGF beta-induced luciferase-expression. X-ray crystal structure analysis of the human ERK2/FR180204 complex revealed that Q105, D106, L156, and C166, which form the ATP-binding pocket on ERK, play important roles in the drug/protein interaction. These results suggest that FR180204 is an ERK-selective and cell-permeable inhibitor, and could be useful for elucidating the roles of ERK as well as for drug development. (c) 2005 Elsevier Inc. All rights reserved.