GNAS and obesity due to melanocortin 4 receptor deficiency: Furthermore, a missense mutation in Gnas exon 6 (referred to as the Oed-Sml mutation) causes post-natal growth retardation when paternally-inherited in heterozygotes; when maternally-inherited in heterozygotes this mutation result in marked subcutaneous oedema, obesity and increased neonatal mortality [62].