significance Consistent with the regulation of synaptic Ca 2+ responsiveness, heterozygous deletion of the SNAP-25 gene in mice results in a hyperactive phenotype similar to attention deficit hyperactivity disorder (ADHD) . In heterozygous mice, a decrease in hyperactivity is observed with dextroamphetamine (or Dexedrine), an active ingredient in the ADHD drug Adderall . Homozygous deletions