IRX3 and obesity due to melanocortin 4 receptor deficiency: Recent studies further demonstrated that two homeobox genes, IRX3 and IRX5, in the vicinity of FTO directly mediate the effects of obesity-risk variants of FTO on body mass and composition regulation (Smemo et al., 2014; Claussnitzer et al., 2015; Laber et al., 2021; Sobreira et al., 2021).