In addition, methylation differences were identified in regions lying in or near genes (such as DENND2D or PWWP2B) related to phenotypes where tobacco exposure is a key risk factor66–68, and maternal smoking, which increases risk of ADHD in the offspring69–71, was the most frequently prenatal stressful life event reported by participants with ADHD. Here, PWWP2B is linked to attention deficit-hyperactivity disorder.