PIK3CA and breast cancer: Given the clinical benefit of PI3KCA inhibitors in PIK3CA-mutated metastatic BC, the analytical challenges of ctDNA testing and the observation that PIK3CA status could change in 1 out of 10 patients, our results indicate the need of obtaining metastatic biopsies for PIK3CA-mutation analysis but also the possibility of testing of the primary tumor, in case a re-biopsy deemed non-feasible.