ESR1 and breast cancer: However, while not recurrent between patients, many of the observed alterations were consistent with prior reports of acquired mutations in metastatic BC, those enriched in BCBrM, and those detected in ctDNA of CSF from patients with BCBrM, including AKT1, BRAF, EGFR, ERBB2, ESR1, FGFR2, GATA3, PIK3CA, PTEN, and RB1 [5, 13, 14, 25, 26].