TPH1 and major depressive disorder: We also used a case-control study design in the same individuals to investigate a possible association with a susceptibility to depression of the SNPs of five other serotoninergic system genes: 5HTR2A (SNP 6313), 5HTR1B (SNP 6296), HTR2C (SNP6318), TPH1 (SNP 1800532), and TPH2 (SNP 7305115).