Interestingly, human carriers of a single nucleotide polymorphism (SNP) in the brain-derived neurotrophic-factor (BDNF) gene that leads to a valine to a methionine substitution at codon 66 (BDNF Val66Met; rs6265) have increased anxiety-related behaviors [15, 16] as well as impairments in fear extinction learning [17]. Here, BDNF is linked to Anxiety.