SLC6A4 and major depressive disorder: For instance, with regard to depression, much research has focused on interactions between environmental factors and polymorphism of the serotonin transporter gene promoter region (5-HTT), following the aforementioned study of Caspi et al. (2003) demonstrating that individuals with one or two copies of the short allele of the 5-HTT promoter polymorphism exhibited more depressive symptoms, diagnosable depression, and suicidality in relation to stressful life events than individuals homozygous for the long allele.