MAPK1 and colorectal carcinoma: Mid-stage CRC involves three pathways: (1) ligand GNAL (chemical stimulation) binds with receptor OR2H1 to activate the G-protein signaling pathway; (2) ligand ATRA (apoptosis signal) binds with receptor RORB to activate the MAPK/ERK pathway (Setia et al., 2014); and (3) ligand CBLN1 (protein secretion) binds with receptor GRID2 to activate the CREB pathway (Hui et al., 2014).