ESR1 and breast cancer: Triple-negative (estrogen receptor-, progesterone receptor, HER2-negative) HCC1806 breast cancer cells were treated with indirubin-3'-oxime (IO) in comparison to kinase-inactive 1-methyl-indirubin-3'-oxime (MeIO) and several indirubin analogues (Table 1), all at a 1 μM concentration for 24 h, and stained for centrin to visualize individual centrioles (Fig. 1A).