PCSK1 and obesity due to melanocortin 4 receptor deficiency: Food intake drives obesity in strains deficient for melanocortin 2 receptor accessory protein 2, Mrap2-/- (Asai et al., 2013), carboxypeptidase E, Cpe-/- (Alsters et al., 2015; Naggert et al., 1995), proprotein convertase 1, Pcsk1+/- (O'Rahilly et al., 1995; Zhu et al., 2002), or growth differentiation factor 15, Gdf15-/- (Tsai et al., 2013).