Genome organization and location of mutations in T3 and T7. The identities and order of all essential genes and most non-essential genes are the same in both phages. Genes shown in blue function in DNA metabolism (dark blue indicates major roles, light blue lesser roles). Genes in red encode virion proteins (the light red gene is for scaffolding, absent in the mature virion). The three internal core genes have unique functions and identities. Genes shown in gray have other functions or their functions are not known. Asterisks are shown above the genes in which compensatory substitutions were observed in either T3 adaptation and shown below the genes of T7 that experienced compensatory evolution. Overlapping genes are offset (the overlap of helicase and primase and the overlap of the two forms of the capsid gene are each shown as a single gene).