MAPK3 and obesity due to melanocortin 4 receptor deficiency: Bost et al. [25] described that ERK1 knockout mice display reduced adipogenesis, protection from obesity and insulin resistance.[25] However, it is impossible to investigate the role of ERK2 on obesity as the deletion of ERK2 in mice led to embryonic lethality.[26] When an ERK pathway inhibitor is administered in ERK1 knockout mice, no impairment in adipogenesis was observed, suggesting that ERK2 is not required for adipogenesis.[130]