Updated project metadata. The PI3Kalpha-specific inhibitor Alpelisib (BYL719) has been approved for the treatment of metastatic ER+/HER2- breast cancer patients in combination with Fulvestrant. After initial response, patients develop drug resistance and disease relapses. In order to identify signalling pathways contributing to the acquired resistance to BYL719 in breast cancer, we generated BYL719-resistant T47D cells and used them together with the parental cells to perform label-free quantitative phosphoproteomics.