Stability of reference genes in grapevine leaves. Stability value (M) for a set of reference genes is analyzed with geNorm algorithm in (a) GLRaV-3-infected (designated as virus-infected), (b) virus-free and (c) combined (virus-free and virus-infected) samples. Reference genes in the x-axis are ranked from left to right based on average expression stability. The GAPDH gene in the extreme left in all graphs with the highest M value denotes lowest expression stability among the reference genes in all samples. Genes at the extreme right in each graph shows the highest expression stability among the reference genes. See legend for Figure 3 for names of reference genes.