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. The gene discussed is ERBB2; the disease is breast cancer.