DRD4 and attention deficit-hyperactivity disorder: More recent work has confirmed such epistatic gene-gene interactions, for instance, by showing that COMT and DRD3 together modulate behavior in children with ADHD (Fageera et al., 2020) and that DRD4 in combination with COMT modulate the clinical responses to clozapine in schizophrenia patients (Rajagopal et al., 2018).