ESR1 and breast cancer: Exposure to AF4 reduced the number of viable triple-negative (MDA-MB-231, MDA-MB-468), estrogen receptor-positive (MCF-7), and HER2 receptor-positive (SKBR3) breast cancer cells in monolayer cultures in a dose- and time-dependent manner, as indicated by reduced cellular metabolic activity measured by 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays (Figure 1A).