ESR1 and breast cancer: According to Perou et al (2000), all these features are characteristics of the so-called luminal subtype of breast cancer, which shares many molecular similarities with normal secretory epithelia, including a strong expression of cytokeratins typical of mature secretory epithelia (CK8/18), hormone receptor (ERα/PR) positivity and negativity for HER2 and the basal cell cytokeratins CK5/6.