ESR1 and breast cancer: Initially, we selected four breast cancer cell lines: MCF-7 and T47D (ER+, mutant PIK3CA E545K and H1047R, respectively), SKBr3 (HER2+) and MDA-MB-231 (triple negative/basal B, mutant KRAS G13D, BRAF G464V) [20], to determine whether firstly the sensitivities to the MAPK pathway inhibitor trametinib correlate with the activity of the corresponding pathway.