In view of the current state of knowledge, the main objectives of our work were as follows: (1) to measure the levels of mRNA expression in the PBMCs of the SERT in subjects affected by MDD or FM and those with both conditions, and (2) to evaluate the effect of quetiapine on the mRNA expression of the SERT in FM and FM + MDD, as well as the effect of various antidepressants in MDD. The gene discussed is SLC6A4; the disease is major depressive disorder.