Given the paucity of information regarding ER+ breast cancers in BRCA1 mutation carriers, we undertook a study to: determine the clinical factors that predict for ER+ breast cancers in this population; compare the pathologic features of ER+ BRCA1-associated breast cancers with those of ER- BRCA1-associated breast cancers; and perform a case-control analysis to compare the pathologic features of ER+ BRCA1-associated breast cancers with those of ER+ sporadic breast cancers. The gene discussed is ESR1; the disease is breast carcinoma.