ESR1 and breast cancer: For example, either deletion of CDKN2A (p16INK4A) or low-expression of E2F transcription factor 1 (E2F1) was the significant predictor of the response to palbociclib in renal cell carcinoma (RCC) cell lines [24], and ER-positive breast cancer cell lines with high-expression of cyclin D1 were more sensitive to palbociclib [7].