Relative uORF compositional adaption to the translational machinery. Panel A shows the frequency histogram of log2 ratios of uORF to principal ORF start codon adaption scores (AUG-CAI(r)). If a uORF is better adapted than the principal ORF, it will produce a positive log value. Panel B shows that same comparison considering the translation adpation index (tAI) of the complete reading frame in order to calculate the log2 ratio. In both cases, uORFs are considerably less well adapted in general with log2 mean values less than zero.