NTRK1 and obesity due to melanocortin 4 receptor deficiency: For example, disruption of the brain-derived neurotrophic factor(BDNF)-TrkB pathway can lead to hyperphagia and obesity in mice.15 Small-molecule, orally active inhibitors might also be more acceptable and cost effective than monoclonal antibodies,16, 17 but selectivity for TrkA against related tyrosine kinase receptors has proved difficult to achieve.