BDNF and major depressive disorder: To investigate whether the BDNF gene is associated with the development of MDD in young patients, several groups have performed polymorphism analyses using targeted DNA sequencing and identified a human single-nucleotide polymorphism (SNP), BDNF Val66Met, that was associated with the development of adolescent MDD (Lau et al., 2010; Cruz-Fuentes et al., 2014).