CYP2D6 and attention deficit-hyperactivity disorder: ADHD, Attention Deficit Hyperactivity Disorder; CYP2D6, cytochrome P450 family 2 subfamily D member 6; CYP2C9, cytochrome P450 family 2 subfamily C member 9, CYP2C19, cytochrome P450 family 2 subfamily C member 19; HLA-A, major histocompatibility complex, class I, A; HLA-B, major histocompatibility complex, class I, B; UM, ultra-rapid metabolizer; PM, poor metabolizer; CPIC, Clinical Pharmacogenetics Implementation Consortium.