SLC6A3 and post-traumatic stress disorder: Similarly, results investigating the influence of the SLC6A3 VNTR polymorphism on PTSD risk have been equivocal, with, for example, the 9R allele related to an increased risk of PTSD and/or hypervigilance symptoms in three studies [19]–[21], but not in an additional, recent multigenerational study of families exposed to a natural disaster [22].