Dual specificity mitogen-activated protein kinase kinase 4, also known as MAP kinase kinase 4, MAPKK4, JNK-activating kinase 1, MAPK/ERK kinase 4, SAPK/ERK kinase 1, c-Jun N-terminal kinase kinase 1, JNKK, and MAP2K4, is a protein that belongs to the protein kinase superfamily, STE Ser/Thr protein kinase family and MAP kinase kinase subfamily. MAP2K4 / JNKK1 is a protein kinase that is a direct activator of MAP kinases in response to various environmental stresses or mitogenic stimuli. MAP2K4 / JNKK1 has been shown to activate MAPK8 / JNK1, MAPK9 / JNK2, and MAPK14 / p38, but not MAPK1 / ERK2 or MAPK3 / ERK1. MAP2K4 / JNKK1 is phosphorylated, and thus activated by MAP3K1 / MEKK. The stress-activated protein kinase (SAPK) pathways represent phosphorylation cascades that convey pro-apoptotic signals. The mitogen-activated protein kinase kinase (MAPKK) homolog MAP2K4 ( MKK4, SEK, JNKK1 ) is a centrally-placed mediator of the SAPK pathways.