The fact that both miR-9 mature transcripts may play different roles in breast carcinogenesis is further demonstrated by the differential expression in the breast cancer subgroups identified by the surrogate molecular classification showing a significant upregulation of miR-9-5p in both triple negative and HER2 amplified breast cancers as compared with luminal subtypes, but a miR-9-3p up-regulation limited to triple negative tumours as compared with luminal and HER2 amplified tumours. The gene discussed is ERBB2; the disease is breast cancer.