The article describes the identification and characterization of PD 098059, a synthetic inhibitor of the mitogen-activated protein kinase (MAPK) pathway. The compound selectively inhibits the MAPK-activating enzyme, MAPK/ERK kinase (MEK), without significantly affecting MAPK itself. In vitro and in vivo experiments demonstrate that PD 098059 prevents the activation of MAPK and subsequent phosphorylation of its substrates, as well as inhibiting cell growth and reversing the ras-transformed phenotype in mouse fibroblasts and rat kidney cells. The study also shows that PD 098059 is a reversible inhibitor, acting through an allosteric mechanism, and is highly selective for MEK. These findings highlight the importance of the MAPK pathway in growth and the potential of PD 098059 as a tool for further research on the role of the MAPK cascade in various biological processes.The article describes the identification and characterization of PD 098059, a synthetic inhibitor of the mitogen-activated protein kinase (MAPK) pathway. The compound selectively inhibits the MAPK-activating enzyme, MAPK/ERK kinase (MEK), without significantly affecting MAPK itself. In vitro and in vivo experiments demonstrate that PD 098059 prevents the activation of MAPK and subsequent phosphorylation of its substrates, as well as inhibiting cell growth and reversing the ras-transformed phenotype in mouse fibroblasts and rat kidney cells. The study also shows that PD 098059 is a reversible inhibitor, acting through an allosteric mechanism, and is highly selective for MEK. These findings highlight the importance of the MAPK pathway in growth and the potential of PD 098059 as a tool for further research on the role of the MAPK cascade in various biological processes.