ESR1 and breast carcinoma: The breast cancer estrogen receptor-positive MCF-7 cells showed a higher sensitivity than the triple-negative MDA-MB-231 cells towards palbociclib in the proliferation assay (Figure 2A), while the non-cancerous cells showed significantly increased resistance (significantly higher IC50 values) when compared to the breast cancer cell lines (Table 1 and Supplementary Materials Table S1).