The aim of this study was to identify the most stable endogenous control genes from a panel of eleven candidates commonly used as endogenous controls in the context of, but not limited to, breast cancer: GAPDH, TFRC, ABL, PPIA, HPRT1, RPLP0, B2M, GUSB, MRPL19, PUM1, PSMC4, for the quantification of gene expression by relative comparative RQ-PCR in primary breast cancer tissues. The gene discussed is GUSB; the disease is breast cancer.