GeNorm analysis of the 12 candidate RGs. Selection of the RGs most suitable for normalization in lung cancer gene profiling studies by GeNorm analysis. The results are presented according to the output file of the program. (a) stepwise exclusion of the least stable genes. The x-axis from left to right indicates the ranking of the genes according to their expression stability, while the Y-axis indicates the stability parameter M. (b) determination of the optimal number of RGs for normalization.