MAOA and attention deficit-hyperactivity disorder: There is some genetic evidence that the X-linked gene MAOB (Xp11.23) encoding the enzyme monoamine oxidase B (which is more selective than monoamine oxidase A and preferentially modulates degradation of dopamine), may also influence attention/impulsivity phenotypes and vulnerability to ADHD (Li et al., 2008), although the evidence that this gene may influence impulsivity is less strong than for MAOA. However, low levels of monoamine oxidase B activity in platelets have been associated with increased sensation seeking behaviours and impulsiveness (Oreland et al., 2002).