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English: Intracellular tumorigenic signaling pathway of neurofibromin and mechanism of action of MEK inhibitors. Neurofibromin promotes the conversion of active GTP-bound RAS to the inactive GDP-bound conformation (left). Loss of neurofibromin increases RAS activity and consequently the signaling cascade of the MEK/ERK pathway. MEK inhibitors (selumetinib, mirdametinib, trametinib, and binimetinib) inhibit this protein and block the MAPK signaling cascade (right). ERK, extracellular signal-regulated kinase; GDP, guanidine diphosphate; GRB2, growth factor receptor-bound protein 2; GTP, guanidine triphosphate; MAPK, mitogen-activated protein kinase; MEK, MAPK/extracellular-signal-regulated kinase; NF1, neurofibromin; Raf, serine/threonine-protein kinase; SOS, son of sevenless.