Determination of the optimal number of control genes. The geNORM program was used to analyze the pairwise variation between the normalization factor NFn and NFn+1. Every bar represents change in normalization accuracy when stepwise adding more endogenous controls according to ranking in Fig. 1. Data were analyzed for the three different time points 4 days (A), 7 days (B) and 21 days (C) after ACA. The use of the two most stable genes is in each case sufficient for an accurate normalization (cutoff 0.15 according to ). The higher V9/10 is due to the inclusion of a relative unstable gene, B2m, and is in accordance with the average expression stability M.