NTRK2 and obesity due to melanocortin 4 receptor deficiency: Mutations in BDNF and its receptor, tropomyosin receptor kinase B (TrkB), are associated with hyperphagia and obesity in mice and humans (Rios et al., 2001; Xu et al., 2003; Yeo et al., 2004; Gray et al., 2006; Han et al., 2008).