Percentage of microbial ORFs having homologs in viruses for 277 prokaryote genomes. The y-axis shows each of the 277 genomes, grouped according to NCBI's taxonomy classification. For each genome, two percentages are shown: red corresponds to ORFans-VH% (percentage of ORFans having homologs in viruses) and blue corresponds to non-ORFans-VH% (percentage of non-ORFans having homologs in viruses). The major 24 clade names are shown, with the number of organisms in each clade shown in parenthesis. The 24 phylogenetic clades are alternately marked by grey and no background colors. For the species names and taxonomies, please refer to Additional file 1. The inset shows the average percentage values of ORFans-VH% and non-ORFans-VH% in various groups. "Total" corresponds to the averages in all 277 genomes taken together, "Non_Firm_Gamma (Others)" corresponds to the averages in the 148, non-Firmicutes, non-Gamma-proteobacteria genomes and "Firm" corresponds to the 66 Firmicutes in the database. The remaining groups in the inset correspond to the major clades (with at least 10 genomes). The figure clearly shows that except for some Firmicutes, ORFans-VH% is much smaller than non-ORFans-VH%, suggesting that the current evidence from homology supporting the hypothesis that the origin of ORFans is viral is weak at best.