MAPK3 and cancer: The main downstream signaling pathway of EGFR/HER2 includes the rat sarcoma (RAS)-RAF proto-oncogene serine/threonine protein kinase-mitogen-activated protein kinase kinase 1/2 (MEK1/2)-extracellular signal regulated kinase 1/2 (ERK1/2) pathway controlling cancer development and progression 5-7.