The reported results for serotonin were varied: hypermethylation of 5-hydroxytryptamine serotonin 1A receptor (5-HTR1A) in SZ and BD in two studies [49, 51], a lack of differential methylation of 5-hydroxytryptamine serotonin 2A receptor (HTR2A) in SZ and BD [50], differential methylation of serotonin 2A receptor (SLC6A4) in BD [48], differential methylation of 5-hydroxytryptamine serotonin receptor 1E G protein-coupled (HTR1E) in SZ [37] and a lack of differential methylation of serotonin neurotransmitter transporter (5-HTT) in SZ [34]. This evidence concerns the gene HTR2A and Behcet disease.