A de novo missense mutation in a key tyrosine residue of the TrkB kinase domain or loss of a functional Bdnf allele has been found to be associated with hyperphagia, severe obesity, and impaired cognitive function in children (Yeo et al., 2004; Gray et al., 2006). The gene discussed is NTRK2; the disease is obesity due to melanocortin 4 receptor deficiency.