POMC and obesity due to melanocortin 4 receptor deficiency: The ablation of POMC neurons or the depletion of the POMC gene induces marked obesity in animals (Gropp et al., 2005; Smart et al., 2006; Xu et al., 2005; Yaswen et al., 1999), while the optical activation of POMC neurons through channelrhodopsin-2 (ChR2) selectively expressed in these neurons diminishes feeding behavior (Aponte et al., 2011).