With the advent of genome-wide association studies, multiple genes and loci such as FTO, TMEM18, CADM1, CADM2, and NEGR1 have been associated with increased susceptibility to common, polygenic obesity; (ii) “syndromic obesity”, which except for obesity is characterized by neurodevelopmental delay or dysmorphic features such as Prader–Willi and Bardet–Biedl syndrome and “monogenic obesity”, which is typically rare, early-onset, severe, inherited in a Mendelian pattern, and results from single gene mutations with large effects. The gene discussed is TMEM18; the disease is obesity due to melanocortin 4 receptor deficiency.