In 2019, Mathews et al. proposed a classification of breast tumors into seven classes, defined by mRNA signatures: Basal/HER2, Basal/Myoepithelial (Myo), Myo/LumA, Myo/LumB, LumA, LumB/Basal, and Myo/LumB/HER2 [301,308]. The gene discussed is ERBB2; the disease is breast neoplasm.