ESR1 and breast cancer: Thus, we report on the in vitro antitumor study of ferruginol (1), ferruginol analogues 2 and 6, tanshinone analogues 7–8, and carnosol analogues 9–10 (Figure 1 and Scheme 1) against different subtypes of human breast cancer cell lines, including the triple negative breast cancer (TNBC) models (lacking estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 overexpression) SUM149 and MDA-MB231, and the two human hormone-dependent cell lines that represent the luminal A subtype of breast cancer, T47D and MCF07.