ESR1 and breast cancer: Previously a patient with metastatic estrogen receptor- positive, HER2-negative breast cancer with CDKN2A loss responded to palbociclib, as briefly described above, which is consistent with the results of the present study, where palbociclib inhibited the Ewing's sarcoma PDOX, which lost CDKN2A, by targeting CDK4/6 [33].