In clinical cancer susceptibility genetics, genes associated with breast cancer are deemed to be of high penetrance if the association between pathogenic variants in that gene and phenotype is typically of OR≥4 (eg, BRCA1, BRCA2), while genes for which pathogenic variants are typically of effect size (OR) 2–4 are deemed to be of moderate penetrance (eg, CHEK2). This evidence concerns the gene CHEK2 and cancer.