SLC6A4 and major depressive disorder: Endeavoring to determine the contribution of APOE, HSPA1A, SLC6A4, BDNF, and HTR2A genetic variants to MDD development, Kitzlerová et al. [23] performed a case-control study with 68 MDD inhabitants of the Czech Republic, in which 53 had the GG (Val/Val) genotype (77.9%), 11 the AA (Met/Met) genotype (16.6%), and 4 the GA (Val/Met) genotype (5.9%).