FGF13 and obesity due to melanocortin 4 receptor deficiency: There are a number of fascinating genes we have consistently implicated in the disorder, like PAX6 which may contribute to some of the vision phenotypes reported in PWS patients (82); IRX5 which has been implicated in obesity and metabolism (83); and FGF13 (formerly referred to as FHF2) which is contained within a region on the X chromosome where aberrations cause strikingly similar Prader-Willi-like phenotypes such as hypotonia, failure-to-thrive, developmental delay, intellectual disability, and in some cases reproductive system anomalies and obesity (84,85)).