MP2 was previously observed as enriching for pCR in HR+HER2– patients treated with IO and other treatment classes18,40,41 and is being prospectively tested in an ongoing randomized trial S2206 (ClinicalTrials.gov identifier: NCT06058377) conducted by the National Cancer Institute Cancer Therapy Evaluation Program and SWOG as a patient selection method for neoadjuvant IO in ER+HER2– cancers. The gene discussed is ESR1; the disease is cancer.