Extracellular signal-regulated kinase 1 (ERK1/p44) is also known as mitogen-activated protein kinase 3 (MAPK3). ERK1 is part of a signal transduction pathway that begins with the small GTPase Ras and continues through a cascade of protein kinases that include Raf and MEK. Activation of this signaling pathway influences many cellular processes such as proliferation, differentiation and survival. ERK1 is very similar in sequence to ERK2/p42 and the activities of both ERK1 and ERK2 are usually considered together as total ERK activity. More recent evidence has shown that ERK1 and ERK2 are not functionally redundant and may have very different and specific roles.