Figure 2. The pan-genome of a) Archaeoglobus, b) Desulfovibrio, and c) Desulfotomaculum can be subdivided into three categories: (i) the core genome (the set of genes shared by all genomes), (ii) the accessory genome (the set of genes present in some but not all genomes), and (iii) the unique genome (the set of genes that are unique to a single genome). The function of each gene in a group is classified using COGs.