MAPK3 and obesity due to melanocortin 4 receptor deficiency: Erk1-/- exhibited a low preference for dietary fatty acids and developed obesity. They also showed higher phosphorylation of MEK, an upstream regulator of ERK1/2 and exhibited high ERK2 phosphorylation, high lipogenesis, and low fatty acid oxidation. Overall, ERK1 and ERK2 have different but key roles in obesity.