There are indications of alterations in the serotonergic system contributing to depression in patients with AD, such as single nucleotide polymorphisms (SNPs) in 5-HT2A and 2C receptors as well as in the serotonin transporter (SERT), along with changes in the expression levels of 5-HT1A, 2A, 2C, and SERT. This evidence concerns the gene SLC6A4 and major depressive disorder.