The MAPK pathway begins with the activation of receptor tyrosine kinases (RTKs) or G-protein-coupled receptors (GPCRs), followed by a cascade involving: 1. RAS - a small GTPase that acts as a molecular switch. 2. RAF - a serine/threonine kinase that activates MEK. 3. MEK - a dual-specificity kinase that activates ERK. 4. ERK - a kinase that translocates to the nucleus to regulate gene expression.