Extracellular signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase family. ERK5 is twice the size of the ERK1/2, containing a conserved amino terminal kinase domain that is 53% identical to ERK1/2, and a unique carboxyterminal region which contains potential binding sites for signalling molecules such as CRK, PI3 kinase and SRC. The second proline-rich region may interact with actin, targeting the kinase to a specific location in the cell. In contrast to ERK1 and ERK2, which are activated by neurotrophins (NTs), cAMP, and neuronal activity in neurons, ERK5 appears to be activated only by neurotrophins. The small GTPase Rap1 and the MEKK2 or MEKK3 kinases are critical upstream signaling molecules mediating neurotrophin stimulation of ERK5 in neurons. MEKK2 or MEKK3 activate MEK5, which appears to be localised in intracellular vesicles. MEK5 then activates ERK5. Once phosphorylated, ERK5 translocates to the nucleus.