FTO and obesity due to melanocortin 4 receptor deficiency: The fat-mass and obesity-associated (FTO) rs9939609 polymorphism is highly associated with obesity-related metrics across the lifespan (Frayling et al., 2007) and facilitates weight gain by increasing energy-dense food intake (Cecil et al., 2008; Church et al., 2010; Gilbert-Diamond et al., 2017; Haupt et al., 2009; Speakman et al., 2008), including in children without obesity (Ranzenhofer et al., 2019).