Patients with BC BrMs and isolated CNS progression at the time of ctDNA were more likely to have ESR1 and PIK3CA mutations (67% and 50%) compared to those with concurrent extracranial/intracranial progression (54% and 46%), or extracranial progression only (18% and 27%, P = .09 and P = .55, respectively). This evidence concerns the gene ESR1 and breast cancer.