Given the mixed response, the patient underwent a second biopsy of a progressing lesion, liver 4 segment 5/6 (L4 seg5/6) (Study biopsy #2, Fig. 1b), revealing metastatic breast carcinoma, ER/PR negative, HER2-normal (1+) by IHC, consistent with triple-negative breast cancer (TNBC). The gene discussed is ESR1; the disease is triple-negative breast carcinoma.