MAPK3 and fibrosarcoma: Activation of tropomyosin receptor kinase A (TrkA), a high-affinity NGF receptor, stimulates the rapidly accelerated fibrosarcoma (RAF)/mitogen-activated protein kinase 1/2 (MEK1/2)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway and activates transcription7.