In this large nationwide nested case–control study, there was a 20% increased odds of breast cancer in women with schizophrenia, schizoaffective disorder, other nonaffective psychotic disorders, and bipolar disorder who used prolactin-increasing antipsychotics for 1–4 years, and a 47% increased odds in those with cumulative use of 5 years or more. The gene discussed is PRL; the disease is psychotic disorder.