A positive association was observed between ESR1 mutation and previous endocrine sensitivity (χ2P = .015), previous aromatase inhibitor exposure (χ2P = .002), bone metastases (χ2P = .005), and a number of all previous lines of treatment for metastatic disease (Cochran-Armitage P = .02), associations similar to those previously reported using digital PCR analysis (Supplementary Figure 7, available online) (22). The gene discussed is ESR1; the disease is metastatic neoplasm.