ESR1 and ductal breast carcinoma in situ: To examine DCIS heterogeneity among various breast cancer molecular subtypes, we classified the lesions by molecular subtype, i.e. luminal A-like (ER+ and/or PR+, HER2−), luminal B-like (ER+ and/or PR+, HER2+), HER2+-like (ER−, PR−, HER2+), and basal-like/triple negative (ER−, PR−, HER2−), for each individual DCIS lesion based upon our ER, PR and HER2 staining results.