Determination of the optimal number of reference genes for accurate normalisation. geNorm calculates pairwise variations (V
) between consecutively ranked normalisation factors NF
is the geometric mean of the expression values for the i first-ranked candidate reference genes). Each pairwise variation value is compared to the cutoff value (0.15, horizontal dotted line). For example, V2/3 corresponds to the pairwise variation between the normalisation factors of the two first-ranked (2) and the three first-ranked (3) reference genes.