A key relay in the cell’s growth signaling chain—when activated, it tells the cell to grow, divide, or survive.Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are serine/threonine kinases in the Ras-Raf-MEK-ERK (MAPK) cascade. Activated by receptor tyrosine kinases and GPCRs, ERK1/2 phosphorylates transcription factors controlling cell proliferation, differentiation, survival, and migration. ERK1/2 activation is a common downstream effect of peptide receptor signaling—including GHS-R1a (ghrelin, ipamorelin), melanocortin receptors, and growth factor receptors. Sustained versus transient ERK activation can produce opposing cellular outcomes.