Several genetic-association studies conducted in recent years have consistently shown that the variants of the fat mass and obesity associated gene, FTO, are significantly associated with body mass index (BMI, kg/m2, see Fig. 1) and fat percentage variation (Frayling et al., 2007; Kilpeläinen et al., 2011) and with other obesity-related phenotypes (Fawcett & Barroso, 2010; Hertel et al., 2011). This evidence concerns the gene FTO and obesity due to melanocortin 4 receptor deficiency.