SLC6A4 and major depressive disorder: We also observed a significant decrease in the SERT gene expression level in the MDD group when contrasted to the two other subject groups: FM by a factor of −1.279 ± 0.380 cycles (p < 0.01), and −1.768 ± 0.342 for FM + MDD (n = 117) (p < 0.001), respectively.