Monoclonal antibodies to K5 and K5/6 have been used to identify basal-like triple negative breast cancers [17, 18] and high K5/6 expression was found to be associated with an increased risk of breast cancer relapse and death [17, 19, 20]. Focal K5/6 expression has also been described in adenocarcinomas of the endometrium, pancreas and ovary [21, 22]. In addition, K5+ subpopulation of cells have been identified in ER+ PR+ luminal breast cancers [23, 24] and are increased in patients whose luminal breast cancers develop resistance to endocrine treatment and chemotherapy [25, 26].