Increased expression of FTO probably underlies the obesity phenotype, as transcripts of the risk allele are produced more abundantly than the non-risk allele.[3], [4] Indeed, mice with a constitutive knock-out of Fto[5]–[7], or with a partial loss-of-function mutation in Fto[8], display a lean phenotype. Here, FTO is linked to obesity due to melanocortin 4 receptor deficiency.