The interplay of genetic variations with environmental influences is widely recognized as the causal mechanism behind depression (Lohoff, 2010). The SLC6A4 gene that codes for the serotonin transporter (5-HTT) is especially expressed in the cortical and limbic area; therefore, its allelic variants have been involved in behavior and depression disorders. Among these variants, there are two located 1400 bp upstream of the transcription start site; a short (S) and a long (L) variant of 14 and 16 copies, respectively, of a 44-bp repeat element (Collier et al., 1996). Current information suggests that these alleles regulate the 5-HTT expression differently, especially the S allele, which may play a key role in the etiology of depressive disorder (Goldman et al., 2010).Facultad de Medicina e Ingeniería en Sistemas Computacionales, Universidad Autónoma de Tamaulipas, H. Matamoros Tamaulipas, Centro Universitario de los Altos, Universidad de Guadalajara. Doctorado en Gene´ tica Humana, CUCS, Universidad de Guadalajara, División de Genética, CIBO, Instituto Mexicano del Seguro Social (IMSS), Guadalajara Jalisco, Hospital General de Sub Zona de Medicina General No. 33, Instituto Mexicano del Seguro Social (IMSS). Delegación Veracruz Norte, San Andrés Veracruz, México Department of Biological Sciences, Center of Biomedical Studies, University of Texas at Brownsville, Brownsville, Texas, USA Facultad de Medicina e Ingeniería en Sistemas Computacionales, Universidad Autónoma de Tamaulipas. Matamoros, Tamaulipas, México