To select the optimal set of reference genes, expression values of the candidate genes were submitted to analysis by the geNorm and NormFinder applications. Table 1 reports the expression stability values (M) of the candidate reference genes in control, disease-stressed and combined sheep groups as calculated by the geNorm applet. The M values are used to rank genes on the basis of their stability: high M values indicate increased gene expression variability, whereas the most stable genes should exhibit M values <1.5 [1] . All studied genes reached acceptable stable expression with low M values, less than 1.5. Based on M value ranking, SDHA appeared to be the most stably expressed gene in control sheep with an average M value of 0.316, followed by YWHAZ and HPRT. In disease-stressed sheep, the overall stability of the candidate genes was lower and YWHAZ was the most stably expressed gene with an average M value of 0.624, followed by GAPDH and SDHA. When data from the two groups were combined, SDHA and YWHAZ To determine the optimal number of reference genes needed to calculate a normalization factor (NF), geNorm measures the pairwise variation between two sequential NFs with an increasing number of reference genes. A cut-off value of 0.15 is usually considered acceptable; it indicates that the control gene combination ensures satisfactory stability and that an additional gene need not be included. In the panel of candidate genes studied here, the use of two genes as references proved to be sufficient for accurate normalization in all sheep groups (Figures 1B, 2B and 3B). 