Moreover, PETstudies in subjects with obesity have shown reduced μ-opioid receptor (MOR)availability (Joutsa et al.,2018; Karlsson etal., 2015, 2016), and it has been proposed that overeating leads to overstimulationof the MOR and concomitant downregulation. This evidence concerns the gene OPRM1 and obesity due to melanocortin 4 receptor deficiency.