[ESR1 Mutation-Positive Recurrent Breast Cancer Successfully Treated with Letrozole plus Abemaciclib].

Approximately 70% of breast cancers are estrogen receptor(ER)positive, an indication for endocrine therapy. The first choice of treatment for ER-positive metastatic recurrent breast cancer is endocrine therapy, which has relatively few side effects; however, many of these side effects become resistant during treatment. One of the resistance mechanisms is mutations in the ESR1 gene, which have also been found to occur after long-term aromatase inhibitor(AI)treatment. Here, we describe our experience of a case in which long-term PR was achieved with AI(letrozole)plus abemaciclib despite ESR1 mutation positivity in cancer genetic screening and review the literature.