The most important biomedical or veterinary muscle conditions (such as muscle hypertrophic growth [24], muscle wasting or atrophy [25], changes in fibre type composition affecting muscle performance [26] or even meat quality [27], and obesity-related insulin resistance [28, 29]) all involve skeletal muscle in the postnatal fibre state. Here, INS is linked to obesity due to melanocortin 4 receptor deficiency.