MC4R and obesity due to melanocortin 4 receptor deficiency: Within the melanocortin pathway homozygous or compound heterozygous mutations have been identified in the PCSK1 gene [8], heterozygous or homozygous loss-of-function mutations in MC4R are the most common cause of severe monogenic obesity [9, 10], while heterozygous inactivating mutations in the GNAS gene encoding the stimulatory G alpha protein (Gαs), by which MC4R signals, also cause obesity [11].