ESR1 and cancer: Furthermore, using a hybridization capture-based NGS assay, known as the Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT) assay8, ESR1 mutations have been detected in 3.5% (11 of 313) of primary breast cancer and 13.6% (84 of 616) of metastatic tumor samples9.