We collected data on patients with schizophrenia (case group) in two waves, which we call (a) CLOZUK and (b) CardiffCOGS (Cardiff Cognition in Schizophrenia). The CLOZUK sample (n = 6558) consists of individuals taking the antipsychotic clozapine. In the UK, clozapine is reserved for patients with treatment-resistant schizophrenia, who provide regular blood samples to allow detection of adverse drug effects. Through collaboration with Novartis, the manufacturer of a proprietary form of clozapine, we acquired anonymised blood samples from people on clozapine. Participants (71% male) were aged 18-90, with a recorded diagnosis of treatment-resistant schizophrenia according to the clozapine registration forms completed by their psychiatrists. The use of these anonymised samples for genetic association studies was approved by the local ethics committee. The CLOZUK sample has been described elsewhere.21 (link) The CardiffCOGS (n = 571) is a sample of patients with clinically diagnosed schizophrenia recruited from community, in-patient and voluntary sector mental health services in the UK. Interview with the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) instrument22 (link) and case-note review was used to arrive at a best-estimate lifetime diagnosis according to DSM-IV criteria.23