ESR1 and breast cancer: In the current study, a series of twenty four feline mammary adenocarcinomas were evaluated according to the current human histologic breast cancer classification, and the TNBC immunophenotype and basal-like subtypes were identified by immunohistochemical expression of ER, PR, and HER2, as well as basal cytokeratins CK5/6 and EGFR.