MAP2K4 and neoplasm: In addition, RAs can block MAPK signaling pathway; inhibit the phosphorylation of p38 mitogen-activated protein kinase (p38), extracellular signal-regulated kinase (ERK), and c-jun n-terminal kinase (JNK); and block the activation of its upstream regulator mitogen-activated protein kinase kinase 4 (MKK4), thus inhibiting the malignant transformation and proliferation of tumor cells [76].