Table 1. Eligibility of cases and controls.

Cases	Controls
Women aged 20 years and over at diagnosis with invasive breast cancer (identified by linkage to the Norwegian Cancer Registry)	Women alive without cancer (other than basal or squamous cell skin cancer) at the time the case was diagnosed, aged in the same 5-year age group as the case
Available stored blood sample taken at least 4 years before the diagnosis of breast cancer (the ‘index sample’)	Available stored blood sample taken within ±2 months of the case's index sample
Earliest available stored blood sample (taken at least 1 year earlier than the index sample described above)	Available early sample (preferably taken within ±2 months of the case's sample, but otherwise the closest early sample taken at least 1 year earlier than the sample described above)