Relative quantity of oestrogen receptor alpha mRNA ( ESR1 ) in benign and malignant breast tumour tissue. Quantity of gene expression was calculated relative to each candidate endogenous control gene and to the geometric mean of MRPL19 and PPIA (QRel. = E-ΔΔCt ± s.e.m.). A pool of cDNA from two normal tissues was used as calibrator. There was no effect of EC on the relative quantity of ESR1 in either group (P > 0.05), however, there was a significant effect of EC gene on the estimate of the error associated with relative gene expression (P < 0.05). The error was significantly reduced using the combination of MRL19 and PPIA compared to the use of all EC genes individually with the exception of MRPL19.