ERBB2 and ductal breast carcinoma in situ: This conclusion is further supported by the finding of increased ER/PR and/or HER2 expression in IDC-DCIS, suggesting in turn that the pure IDC carcinogenesis pathway could favour the therapeutically challenging basaloid phenotype, which is already known to be associated with multiple defects in tumour suppressor genes such as TP53 and the BRCA DNA repair genes.