This is a randomized, placebo-controlled study to assess the efficacy and safety of ATX augmentation of CLZ as a treatment for the cognitive symptoms of schizophrenia. A total sample size of 126 subjects diagnosed with schizophrenia and being treated with the antipsychotic clozapine will undergo genotyping. These subjects will have an initial assessment at four weeks and regular follow-up assessments for a total period of 52 weeks. These subjects will be randomized to continued treatment with CLZ and augmentation with ATX or Placebo.