PCP4 and attention deficit-hyperactivity disorder: Three genes harbored coding variants that were in strong LD with the peak SNP: Brwd1, Pcp4 and Sh3bgr. In humans, BRWD1 has been associated with potentially relevant traits including cigarette smoking [63–68], externalizing behaviors [69], ADHD [66, 70] and hyperthymia in bipolar disorder [71].