Accession Numbers
Requests for access to this dataset should be made to the BCAC co-ordinator, contact provided in Web Resources.

Web Resources
BCAC data access, http://bcac.ccge.medschl.cam.ac.uk
BCAC Summary statistics, http://bcac.ccge.medschl.cam.ac.uk/bcacdata/oncoarray/gwas-icogs-and-oncoarray-summary-results/
CORDIS, https://cordis.europa.eu/project/rcn/212694_en.html
GenomeCanada 2018 projects, https://www.genomecanada.ca/sites/default/files/2017lsarp_backgrounder_en.pdf
NICE, familial breast cancer clinical guidelines (accessed June 4, 2018), http://guidance.nice.org.uk/CG164
Nomis (26 March 2018), https://www.nomisweb.co.uk/
Office of National Statistics, https://www.ons.gov.uk/
West Midlands Cancer Intelligence Unit, http://www.wmciu.nhs.uk/

Supplemental Data
Document S1. Figure S1, Tables S2–S6 and S9–S12, Supplemental Acknowledgments, and Supplemental Material and Methods
Table S1. Studies and Samples in the Training Set
Table S7. SNPs and Effect Sizes for 313 SNPs Used in the Construction of Overall Breast Cancer and Subtype-Specific PRSs
Table S8. SNPs and Effect Sizes for 3,820 SNPs Used in the Construction of Overall Breast Cancer and Subtype-Specific PRSs
Document S2. Article plus Supplemental Data

Acknowledgments
BCAC was funded by Cancer Research UK (C1287/A16563) and by the European Community’s Seventh Framework Programme under grant agreement no. 223175 (HEALTH-F2-2009-223175) (COGS) and by the European Union’s Horizon 2020 Research and Innovation Programme under grant agreements 633784 (B-CAST) and 634935 (BRIDGES). Genotyping of the OncoArray was principally funded by Government of Canada through Genome Canada and the Canadian Institutes of Health Research (grant GPH-129344), the Ministère de l’Économie, de la Science et de l’Innovation du Québec through Genome Québec, the Quebec Breast Cancer Foundation; NIH grants U19 CA148065 and X01HG007492; and Cancer Research UK (C1287/A10118 and C1287/A16563). Genotyping of the iCOGS array was funded by the European Union (HEALTH-F2-2009-223175), Cancer Research UK (C1287/A10710), the Canadian Institutes of Health Research for the “CIHR Team in Familial Risks of Breast Cancer” program, and the Ministry of Economic Development, Innovation and Export Trade of Quebec (grant # PSR-SIIRI-701). Combining the GWAS data was supported in part by the National Institutes of Health (NIH) Cancer Post-Cancer GWAS initiative grant: No. 1 U19 CA 148065 (DRIVE, part of the GAME-ON initiative). We thank all the individuals who took part in these studies and all researchers, clinicians, technicians, and administrative staff who enabled this work to be carried out. For other acknowledgments and sources of funding, see Supplemental Acknowledgments. Supplemental Data include 2 figures, 12 tables, Supplemental Acknowledgments, and Supplemental Material and Methods and can be found with this article online at https://doi.org/10.1016/j.ajhg.2018.11.002.