ERK1, a MAPK-type protein kinase, is a member of the Ras-MAPK pathway. This pathway modulates the activity of transcription factors, structural proteins, and other protein kinases, leading to biological responses such as cell proliferation and differentiation. Inactive ERK1 associates with MAPK/ERK kinases (MEK) 1 and 2 in the cytosol. Upon stimulation, MEK1/2 phosphorylates and activates ERK1 causing dissociation of the MEK/ERK complex. A portion of the activated ERK1 pool forms dimers and translocates to the nucleus to activate nuclear proteins such as ELK-1 and MSK1. Cytosolic ERK 1 acts on a different set of substrates, including MNK1/2 and MAPKAP-K1.