Mutational signatures of CTCs are consistent with those expected for advanced HR+ breast cancer, including the presence of ESR1 mutations in three of the six lines (Brx50, Brx68, Brx211), and PIK3CA mutations in three of the six lines (Brx68, Brx211, Brx394) (Table 1 and Sup. Here, PIK3CA is linked to breast cancer.