We found some evidence that experiencing insomnia symptoms (compared to no symptoms) was protective against breast cancer specific mortality (i.e. prolonged survival) in patients with ER + /PR + HER2- tumours who received no chemotherapy (HR = 0.18, 95% CI = 0.05, 0.68), as well as among patients given aromatase inhibitors (HR = 0.23, 95% CI = 0.07, 0.78), whilst this was not the case for those receiving other chemotherapies (Additional file 3:1). The gene discussed is CYP19A1; the disease is insomnia.