BDNF and obesity due to melanocortin 4 receptor deficiency: This genetic finding was confirmed and extended by the severe obesity phenotypes observed in mice expressing the TrkB receptor at ~25% of normal levels (Xu et al., 2003) and in mice where the Bdnf gene is deleted in neurons expressing Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIα) (Rios et al., 2001).