 Typically , a number of genes are involved , many of which directly affect dopamine neurotransmission . Those involved with dopamine include DAT , DRD4 , <unk> , TAAR1 , <unk> , COMT , and DBH . Other genes associated with ADHD include SERT , <unk> , <unk> , <unk> , <unk> , <unk> , and BDNF . A common variant of a gene called <unk> is estimated to be responsible for about 9 % of cases and when this variant is present , people are particularly responsive to stimulant medication . The 7 repeat variant of dopamine receptor D4 ( DRD4 – 7R ) causes increased inhibitory effects induced by dopamine and is associated with ADHD . The DRD4 receptor is a G protein @-@ coupled receptor that inhibits <unk> cyclase . The DRD4 – 7R mutation results in a wide range of behavioral phenotypes , including ADHD symptoms reflecting split attention . 
