Background <p>Current treatment strategies for hormone-dependent breast cancers, including adjuvant endocrine therapy, often fail due to persistence of breast cancer stem cells (brCSCs), which are significant contributors to tumor recurrence and treatment resistance. Therefore, gaining deeper insights into the molecular regulators driving breast cancer aggressiveness is important. Moreover, given the complexities and expenses involved in developing new pharmacological agents, the strategic repurposing of existing FDA-approved drugs to target these key molecular pathways presents a compelling approach for identifying novel therapeutic interventions aimed at mitigating tumor refractoriness.</p> Methods <p>The study employs survival analysis from TCGA database, protein expression analyses alongside aldefluor assays, sphere formation efficiency tests to evaluate cellular stemness, and DCFDA analysis combined with antioxidant enzyme assays to investigate redox imbalance in brCSCs. These analyses were conducted following the genetic deletion of YAP/TAZ and pharmacological treatment with verteporfin.</p> Results <p>The study demonstrated that transcriptional co-activators YAP/TAZ are significantly upregulated in chemotreated ER⁺ patient breast tumors and MCF-7 mammospheres, where it was found to interact with the transcription factor SOX2 within the nuclear compartment. Genetic ablation and pharmacological inhibition of YAP/TAZ markedly impaired stemness properties and disrupted redox homeostasis in the mammospheres. Additionally, treatment with verteporfin led to a substantial reduction in the frequency and viability of brCSCs, suggesting their effective eradication.</p> Conclusion <p>This study highlights the potential of repurposing verteporfin, an FDA-approved drug originally formulated for age-related macular degeneration, as a therapeutic agent for targeting YAP/TAZ-mediated stemness and redox balance in brCSCs, thereby reducing their viability in ER-positive breast cancers.</p>