To define ATRA sensitivity, we selected 42 cell lines representative of breast cancer heterogeneity (Supplementary Table S1) and characterized for ER, PR, and HER2 status as well as the Luminal or Basal phenotype according to PAM50 (Tibshirani et al, 2002; Parker et al, 2009) (Supplementary Fig S1). The gene discussed is ERBB2; the disease is breast carcinoma.