Gene expression stability of the 6 candidate reference genes analysed by the geNorm program . (a) Average expression stability values (M) of the control genes, plotted from least stable (left) to most stable (right). (b) Pairwise variation analysis between the normalisation factors NFn and NFn+1, to determine the optimal number of control genes for normalisation. The higher V3/4 value is due to the inclusion of a relative unstable gene and is in accordance with the average expression stability M. A – Normal skin. B – Equine sarcoids. C – Combination of normal skin and equine sarcoids.