KSR1 is a molecular scaffold and positive regulator of the Raf/MEK/ERK phosphorylation cascade. It was initially considered to be a protein kinase acting in the Ras pathway. KSR1 is required for maximal ERK activation induced by growth factors and by some cytotoxic agents. In resting cells, phosphorylated KSR1 is cytoplasmic and in stimulated cells, dephosphorylated KSR1 is membrane associated.