ABCB6 and major depressive disorder: For example, polymorphisms in ABCB1 and ABCB6 (e.g., rs1109866, rs1109867) have been linked to MDD risk and poorer cognitive performance, suggesting that these genes affect emotional regulation through neurocognitive pathways [28], whereas ABCB1-rs1045642 has been identified as a protective variant for both MDD and bipolar disorder [29].