Phylogenetic tree for selected B-type cyclins. The B3 cyclins form an evolutionarily conserved family distinct from all the other B-type cyclins. The number of these other B-type cyclins varies between organisms. In vertebrates there are two evolutionarily divergent groups; one includes the B1 and Xenopus (frog) B4 cyclins and the other the B2 and Xenopus B5 cyclins. The alignment used for the construction of this phylogenetic tree is given in the additional data files. O. latipes, Oryzias latipes. The scale bar represents 0.1 substitutions per site.