Selection of the most suitable reference genes for normalization in rhesus monkey (a and b) and human (c and d) samples using geNorm analysis. Stepwise exclusion of the least stable genes by calculating the average expression stability (a and c) was conducted. The value of M was calculated for each gene, and the least stable gene with the highest M value was automatically excluded for the next round of calculations. The x-axis from left to right indicates the ranking of the genes according to their expression stability. Determination of the optimal number of reference genes for normalization (b and d) was conducted. The software calculates the normalization factor from at least two genes at which the variable V defines the pair-wise variation between two sequential normalization factors.