Since mutations in the gene for TrkB, but not p75NTR, lead to obesity in humans and mice (Lee et al., 1992; Xu et al., 2003; Yeo et al., 2004), BDNF should act on the TrkB receptor to mediate its effects on appetite and energy expenditure. This evidence concerns the gene BDNF and obesity due to melanocortin 4 receptor deficiency.