DRD4 and attention deficit-hyperactivity disorder: For example, multiple candidate-gene association studies have indicated that polymorphisms in DRD4 and DAT1 are associated with cADHD (Brookes K. et al., 2006; Gornick et al., 2007), while results from a case-control study did not find any association between these two genes and ADHD (Johansson et al., 2008).