Pharmacological treatment of schizophrenia is still characterized by a certain proportion of patients who are resistant to common antipsychotics. Variability in individual response ranges from patients who experience complete or near complete symptom remission to a subset of patients who remain treatment-refractory despite extensive drug trials over years. The investigators propose to take advantage of the simultaneous acquisition of clinical and of intermediate phenotypes to ascertain the possible contribution of SNPs within putative causative genes to response to treatment with quetiapine XR.