In order to demonstrate that FKBPL-based clinical peptide, ALM201, is able to target endocrine therapy resistant CSCs in ER+ breast cancer, we treated the ER+ breast cancer cell line, MCF-7, with estradiol (100 nM) ± tamoxifen (1 μM) ± ALM201 (1 nM) and carried out mammosphere assay. This evidence concerns the gene ESR1 and breast cancer.