Targets fold change homogeneity. PCA and agglomerative hierarchical clustering were used to describe the homogeneity degree of target fold change variation as a function of the reference used. Expression levels of three target genes (RRM2, BRCA1, ERCC2) were normalized with respect to each of the RGs and expressed as -ΔΔCt using normal paired samples as calibrator. A) Small fluctuations in fold change target detection using reliable RGs results in a very limited spread over the PCA space. As expected, POLR2A, rRNA18S, ESD and YAP1 produce the best homogeneous cluster (filled dots). B) Similar results are obtained by hierarchical clustering, where the smallest Euclidean distance is associated with the previously indicated set of genes.