The opinion that JNK functions in insulin resistance is supported by the following observations: (1) JNK over-activation is observed during obesity, and mice with a JNK1 deficiency display a significantly improved insulin sensitivity level [72,80]; (2) JNK1 knockout mice are protected from obesity [80,81]; (3) mice lacking JNK2 also display an improved insulin sensitive phenotype [81]; and (4) there are some mutations in the JIP1 coding gene (JNK’s scaffold) in patients with type II diabetes [82]. Here, MAPK9 is linked to obesity due to melanocortin 4 receptor deficiency.