A functional polymorphism (5-HTTLPR) in the promoter (regulatory) region of the serotonin transporter gene (SLC6A4) has been widely studied as a potential predictor of depression liability and stress response (Caspi et al., 2003; Clark, Flint, Attwood, & Munafό, 2010). There is a long (L) and short (S) version (or variant) of the 5-HTTLPR polymorphism. Furthermore, it has been found that the L variant comes in two forms: LA and LG (Hu et al., 2005; Nakamura, Ueno, Sano, & Tanabe, 2000). The frequencies of the S and LG variants range from 0.40 – 0.09 respectively in Caucasians to 0.25 – 0.24 respectively in African Americans. The S and LG variants alter expression of the gene. The S variant is associated with an approximately 50% reduction in transporter availability compared with the L variant; likewise, the same reduction in transporter availability is found when the LG and LA variants are compared (Lesch et al., 1996). There appears to be a weak positive association between the 5-HTTLPR S variant and increased AD risk (Feinn, Nellissery, & Kranzler, 2005). Unfortunately, earlier studies included in
