Estetrol and ShERPA BMI-135 display activity similar to E2 but right shifted across eight BC cell lines that are estrogen-dependent (MCF-7:WS8, T47D:A18, MCF-7:PF, BT-474, and ZR-75-1), estrogen-independent (MCF-7:5C, MCF-7:2A, and MCF-7:RAL), endocrine-sensitive (MCF-7:2A), endocrine-resistant (MCF-7:PF, MCF-7:5C, and MCF-7:RAL), mutant p53 (T47D:A18), human epidermal growth factor receptor 2–positive (BT-474), luminal A (ZR-75-1), and luminal B (BT-474). This evidence concerns the gene ERBB2 and breast cancer.