Iroquois-related homeobox 3 is considered to be a determinant for obesity, in relation to the fat mass and obesity associated (FTO) genes, due to the role of Irx3 in neurogenesis at the paraventricular nucleus of the hypothalamus, developed from the anterior hypothalamus (Smemo et al., 2014). The gene discussed is IRX3; the disease is obesity due to melanocortin 4 receptor deficiency.