As review by Xu and Xie, genetic mutations in human BDNF and its receptor TrkB result in morbid early-onset obesity (Xu and Xie 2016); furthermore, genome wide associated studies (GWAS) have identified single nucleotide polymorphisms (SNPs) in or near BDNF to be associated with increased body mass index (BMI) (Xu and Xie 2016). This evidence concerns the gene NTRK2 and obesity due to melanocortin 4 receptor deficiency.