However, our conclusion is not in disagreement with a basal origin of claudin-low breast cancer, as our data, in particular that from the premalignant lesions (Supplementary Fig. 2a–c), revealed a possible transition of Trp53-null luminal cells to a K14+ and/or K5+ basal-like intermediate state first, before emergence of claudin-low tumours. Here, KRT5 is linked to breast cancer.