A specific genetic factor, an alteration in the 5-HTTLPR gene that regulates serotonin, may act as a diathesis for depression. Research by Caspi et al. (2003) indicates that individuals carrying one or two short versions of this gene are significantly more prone to depressive episodes following multiple stressful life events. Conversely, those with two long versions of the gene appear to be more resilient to the depressogenic effects of stress. This genetic vulnerability alone is not sufficient to cause depression; its impact is contingent upon exposure to stress.