Gene expression profiling based on complementary DNA (cDNA) microarrays led to the classification of breast cancer into distinct subtypes, with separate prognostic and treatment implications: luminal A (LA), luminal B (LB), basal-like (BL), and Her2-enriched (HER2).2,3 The PAM50 assay measures the messenger RNA (mRNA) expression levels of 50 genes that can classify breast cancers into the same subtypes. Here, ERBB2 is linked to breast carcinoma.