NTRK2 and obesity due to melanocortin 4 receptor deficiency: In addition, Ntrk2 heterozygous mice do not show major abnormalities while loss of one NTRK2 (TrkB) allele in human leads to hyperphagic obesity and severe impairment in memory, learning and nociception (Klein et al., 1993; Yeo et al., 2004).