<b>Results:</b> CEU-938 demonstrated marked and selective antiproliferative activity across molecular subtypes of human breast cancer, with efficacy observed in approximately 40% of triple-negative breast cancer (TNBC), 70% of estrogen receptor-positive (ER<sup>+</sup>), and 80% of human epidermal growth factor receptor 2-positive (HER2<sup>+</sup>) breast cancer cell lines, while sparing non-tumorigenic human breast cells (MCF 10A, MCF-12A, 184B5). This evidence concerns the gene ESR1 and breast cancer.