NTRK2 and obesity due to melanocortin 4 receptor deficiency: Interestingly, Ntrk2 or Bndf homozygous knockouts are not viable, but Bndf heterozygous knockouts, as well as conditional deletion of Bndf in the postnatal brain and hypomorphic mice expressing 24% of normal levels of TrkB all exhibit increased food intake and obesity [28], [30]–[32].