The preclinical work that paved the way for testing palbociclib in humans was conducted by Finn and colleagues, who tested its growth inhibitory effects on a large panel of human breast cancer cell lines.638 They identified potent activity in estrogen receptor-positive cells, including those that were HER2-amplified, while basal-like cell lines were the most insensitive to the drug. This evidence concerns the gene ESR1 and breast carcinoma.