Statistical significance in yeast upstream sequences and 5' UTRs. In A), the average number of uORFs is shown for both upstream regions in general (No TSS) and TSS mapped 5' UTRs (Superset). The same number of sequences (4149) is used in all cases for the "No TSS" line. At all points, there are fewer uORFs in the real 5' UTR sequences, In B), a normalised frequency histogram of uORF numbers in real and shuffled 5' UTRs highlights the reduced chance of observing a uORF in real UTR sequence compared to random. Over 85% have none compared to 68% by chance.