Afterwards, several studies conducted similar genome-wide analyses [39,40,41], up until 2009 when Bernard et al. developed a qRT-PCR-based assay using only fifty genes (PAM50) to classify tumors into four intrinsic subtypes of breast cancer: luminal A, luminal B, HER2-enriched, and basal-like [42]. The gene discussed is ERBB2; the disease is breast cancer.