GeNorm analysis of the candidate EC genes. Results are presented as per the output file of the geNorm programme . (a) Stepwise exclusion of the least stable genes. The gene stability value M is based on the average pairwise variation between all tested genes. Low M values characterise genes with greater stability, thus the x-axis from left to right indicates the ranking of the EC genes according to expression stability and the y-axis indicates the stability measure, M. (b) Determination of the optimal number of ECs for normalisation. The recommended upper limit of the pairwise variation value V is set at 0.15 but in meeting this criterion, sample availability, the practicality of using multiple EC genes and the degree of required resolution must be considered.