PGR and breast cancer: To further explore phospho-PR activity in metastatic breast cancer, we employed PDX models established from metastatic ER+/PR+ breast cancer patients.30 ER expression and total or phospho-PR expression were confirmed by IHC using specific monoclonal (total) and previously characterised14 custom-made polyclonal (pS294 PR) antibodies (Fig. 1b, Supplementary Fig. S1).