A model for gene duplication and loss in the Fox cluster loci during vertebrate evolution. Two initial rounds of duplication of the whole region are inferred to have occurred by the base of bony vertebrates, and this is consistent with both the 2R hypothesis and multiple block duplications. Considerable gene loss is then inferred before the radiation of bony vertebrates. Subsequent losses and inversions in different vertebrate lineages are indicated on the tree, with capital letters indicating the paralogon (nomenclature as shown in Figures 1 and 2) affected. A third round of genome duplication (3R) is indicated in the teleosts.