ERBB2 and breast cancer: To determine whether ATRA modulates autophagy in breast cancer cells, we first measured steady-state levels of the autophagy marker LC3B-II in the two luminal, HER2-positive ER-negative breast cancer cell lines, SKBR3 (HER2/RARα coamplification) and MDA-MB453 (HER2 amplification), upon challenge with different concentrations of ATRA during different time periods.