The latter is further supported by studies in animals showing that genetically or pharmacologically induced deficiencies in BDNF or its receptor, tyrosine receptor kinase B (TrkB), cause memory and learning impairments [100,101] and a study in humans showing that functional loss of a copy of the BDNF gene has been linked to hyperactivity, severe obesity, and impaired cognitive function [102]. Here, BDNF is linked to obesity due to melanocortin 4 receptor deficiency.