In duration of use analyses, each year of estrogen-only HRT was associated with a reduction in BC risk overall (HR, 0.90; 95% CI, 0.81-0.99) and a reduction among participants with BRCA1 PV (HR, 0.87; 95% CI, 0.77-0.98).<h4>Conclusions and relevance</h4>In this cohort study of women with BRCA PV who received HRT after RRBO, estrogen-only HRT was not associated with an increased risk of BC and was associated with a lower risk of BC among women with BRCA1 PV. The gene discussed is BRCA1; the disease is breast cancer.