Importantly, these basal-HER2+ breast carcinomas show immunoreactivity for the basal epithelium cytokeratin marker CK5/6, which at times appears as a uniform or almost uniform positive staining that fully mimics the pattern of pure basal-like breast carcinomas, but often displays checkerboard-type intratumoral heterogeneity. Here, KRT5 is linked to breast carcinoma.