The team identified a number of genes with altered methylation in people with PTSD – all of which were involved in immune function and brain cell development. This might explain why PTSD sufferers have weaker immune systems and poorer memories, says Galea. What's more, the number of affected genes was linked to the number of traumatic events a person with PTSD had experienced. "We think that traumatic events induce methylation changes," says Galea. Link thanks to haikujaguar.