We identified a rare variant in the first intron of PLK1 (rs138974428; imputation r2 = 0.95; Europeans minor allele frequency = 0.006) as potentially associated with reduced risk of breast cancer in carriers of pathogenic BRCA1 variants (hazard ratio (HR) = 0.66, P = 0.0045) but not BRCA2 variants (HR = 0.78, P = 0.096) (Figure S18a). This evidence concerns the gene PLK1 and breast cancer.