The 21 genes assay includes 16 cancer genes subdivided into five proliferative genes (Ki67, STK15, Survivin, CCNB1, and MYBL2), five estrogen receptor (ER) genes (ER, PGR, BCL2, and SCUBE2), two HER2 genes (HER2 and GRB7), two invasive genes (MMP11 and CTSL2), and one gene from GTSM1; and remaining five reference genes including B-Actin, GAPDH, RPLPO, TFRC, and GUS genes. The gene discussed is ESR1; the disease is cancer.