Regarding the serotoninergic system, the serotonin transporter gene (SLC6A4) methylation levels are related to traumatic events, child abuse, work stress, and depression [115–121], and although it does not correlate with mRNA expression in the blood of patients, the hypomethylation of SLC6A4 is proposed to be a biomarker of diagnosis and drug response to major depression [118,122]. Here, SLC6A4 is linked to depressive disorder.