SLC6A4 and major depressive disorder: SLC6A4 methylation was previously found to be associated with psychiatric disorders such as depression and alcoholism (Philibert et al., 2008; Devlin et al., 2010) and has been used as a peripheral marker for various neuropsychiatric disorders that involve 5-HT alterations (Hernández et al., 2002; Barkan et al., 2006; Marazziti et al., 2006).