Correlation between d_SM and difference in magnitude of gene expression. We found a significant positive correlation between expression difference and shared motif divergence (d_SM) in sequences 0–500 bp upstream of translation start between duplicate genes in duplicate families with two to five members (n = 76, r_s = 0.47, P < 10^–3; Spearman rank correlation). The log(expression difference) is linearly correlated with d_SM (R = 0.46; P ≪ 10^–3; Pearson linear correlation); a linear fit of the data is also plotted (y = 0.85 + 1.37x). Similar results were obtained with strict duplicate pairs or duplicate gene families of up to 10 members (data not shown).