ESR1 and cancer: In the present study, we used three different types of breast cancer cell lines, MCF-7 (Luminal A; ER+, PR+/−, HER2−, good outcome), SK-BR-3 (HER2; ER−, PR−, HER2+, poor outcome), MDA-MB-231 (Claudin-low; ER−, PR−, HER2−, poor outcome)26 and demonstrated that shikonin inhibits the growth of these cancer cells including arresting the progression of the cell cycle and inducing apoptosis.