ESR1 and breast cancer: Notably, lobular breast cancer (LBC) is less common as a proportion of breast cancers in women with BRCA1 PGVs compared to women without BRCA1 PGVs [3–6] As the main pathway to breast cancer is from basal progenitor cells leading to the predominant ductal triple negative breast cancer, this route excludes the predominantly estrogen receptor positive lobular cancer.