U-value histogram for all the 818,906 ORFs in 277 prokaryote genomes. The U-value is a measure of the "conservation" of each ORF (see Methods); U = 0 means the ORF is unique to one single organism, i.e. a singleton or paralogous ORFan. 9.1% of all ORFs have U = 0. The left tail 0.0 < U <= 0.1 (4.3% of all ORFs) corresponds to orthologous ORFans, ORFs with homologs only in closely related organisms. Notice the uneven distribution of U, with its long left tail and the very high peak at U = 0.