RAF1 (cRAF) is a MAP kinase kinase kinase (MAP3K) which functions downstream of the Ras family of membrane associated GTPases to which it binds directly. Once activated RAF1 can phosphorylate and activate the dual specificity protein kinases MEK1 and MEK2 which in turn phosphorylates and activates the serine⁄threonine specific protein kinases ERK1 and ERK2. The RAF1 Y340D Y341D mutant is constitutively active.