ESR1 and breast cancer: Here we analyzed the efficiency of AEB071 treatment in estrogen receptor (ER) positive (T-47D), ER/Human Epidermal Growth Factor Receptor 2 (HER2) double positive (BT474), and ER/HER2 negative (HCC1806) BC cell lines in 2D (i.e., monolayer) and 3D cell culture, the latter referred to as multicellular tumor spheroids (MCTS).