Another gene/environment interaction pertains to depression, a disease involving serotonin abnormalities.33 A gene called 5HTT codes for a transporter that removes serotonin from the synapse; having a particular 5HTT variant increases the risk of depression ... but only when coupled with childhood trauma.fn23 What's the effect of 5HTT variant on depression risk? It depends on childhood trauma exposure. What's the effect of childhood trauma exposure on depression risk? It depends on 5HTT variant (plus loads of other genes, but you get the point).