SLC6A4 and major depressive disorder: However, despite that multiple studies have tried to identify specific candidate polymorphisms in single genes underlying MDD development, recent studies of multiple large samples have demonstrated that, there was no support for historical candidate gene to have any actual relationships with MDD development, including the most thoroughly investigated gene SLC6A4 (8), and common single-nucleotide polymorphisms (SNPs) account for only less than 30% MDD risk variance (21).