Determination of the optimal number of reference genes for normalization. The GeNorm software calculates the normalization factor from an increasing number of genes (starting with at least two) for which the variable V defines the pairwise variation between two sequential normalization factors. The lower the pairwise variation, the better is the combination of genes for reference. V8/9 for example (in Figure 5a), shows the variation between the normalization factors of eight genes in relation to nine genes and shows that nine genes is the combination providing the lowest pairwise variation. For figure legends: see Figure 4.