Maximum Likelihood (ML) phylogenetic tree based on the nucleotide sequence of a complete atip ORF. The ML tree was constructed with MEGA 5.05 as detailed in Methods. Bootstrap values were determined from 1000 replica sampling and only bootstrap values above 70% are shown. The scale indicates substitution per site. Neighbor Joining (NJ) tree constructed with MEGA 5.05 gave identical tree topologies and similar bootstrap support as the ML trees but Bayesian Inference (BI) tree generated with Mr. Bayes 3.1.2 was poorly resolved. CPXVs (14 in total) belonging to clusters (groups) 1 to 5 are CPXV-like because the descended from the same common ancestor as the reference strain CPX-BR. CPXV group 6 are VACV-like because they are closer to VACV than to CPXV-like CPXVs and share a common ancestor with VACV.