The MAP kinase cascades. This highly simplified scheme shows the three major MAP kinase pathways of mammalian cells. The MAP kinases are ERK, JNK, and p38. A few downstream substrates are shown. ATF = activating transcription factor; cPLA2 = cytosolic phospholipase A2; CREB = cAMP-response-element-binding protein; ERK = extracellularly regulated kinase; JNK = c-Jun N-terminal kinase; LPS = lipopolysaccharide; MAP = mitogen-activated protein; MAPKAPK = MAP kinase-activated protein kinase; MKK = MAP kinase kinase; MKKK = MAP kinase kinase kinase; TNF = tumour necrosis factor.