COMT and schizophrenia: The allelic variants (Val158Met) in the COMT gene, coding for the dopamine-degrading enzyme, a main regulator of the prefrontal dopamine level, increased the prefrontal dopamine catabolism and impaired prefrontal cognition in schizophrenic patients, while the MAO-B rs 1799736 polymorphism (A to G substitution in the MAO-B enzyme with a role in dopamine degradation and the regulation of dopaminergic neuron activity) was suggested as being connected with the aetiology of schizophrenia and negative symptoms development.