To better elucidate the association between the prognosis of ER+/HER2− BC and the CSC surface marker CD133, we utilized two distinct cohorts: The Cancer Genome Atlas (TCGA) cohort, which includes 1065 breast cancer patients, and the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) cohort, consisting of 1904 breast cancer patients. This evidence concerns the gene PROM1 and breast cancer.