Although many publications have now studied the role of the FTO locus in the context of obesity, a number of studies demonstrated that FTO is, in fact, likely not the principal causal effector gene for obesity at this locus, but rather it is IRX3 and IRX5 (11–13), suggesting that the genetic variant resides in an enhancer embedded in one gene that influences the expression of others. Here, IRX5 is linked to obesity due to melanocortin 4 receptor deficiency.