SLC6A3 and major depressive disorder: It has been established that the 9-repeat allele (9R) of SLC6A3 is associated with various mental disorders, including major depressive disorder (Dong et al., 2009; Gontijo et al., 2023), suicidal behavior (Rafikova et al., 2021), and schizophrenia (Sáiz et al., 2010).