ErSO-TFPy rapidlykills ERα+ breast cancer cellsat low nanomolar concentrations in essentially a quantitative fashionand is well-tolerated in multiple species (mice, rats, and dogs).Surprisingly given its relatively short half-life in vivo (SI Figure 1), a single IV dose of ErSO-TFPy is sufficient to induce complete or near-complete tumor regressionsin three different human tumor models of breast cancer in immunocompromisedmice. The gene discussed is ESR1; the disease is neoplasm.