SLC6A4 and major depressive disorder: 2013, Lin et al. 2015, Ramesh et al. 2022, Savitz et al. 2009). 5‐HTR2A (e.g., rs6313) and SLC6A4 (5‐HTTLPR) variants further support serotonergic dysregulation, especially in MDD and SZ (Lin et al. 2015, Petit et al. 2014). Notably, individuals with the l/l genotype of 5‐HTTLPR show better SSRI responses and reduced SZ risk, linking synaptic serotonin transport to clinical outcomes (Lin et al. 2015, Petit et al. 2014, Ramesh et al. 2022).