Although the effect size of variation at the FTO locus is not comparable to the more impactful rare variants in the monogenic forms of the disease, it remains the most established association with common obesity to date, primarily due to its high frequency in the population, where the minor allele frequency of the rs9939609 allele ranges from 38% to 44% in Caucasians. This evidence concerns the gene FTO and obesity due to melanocortin 4 receptor deficiency.