Interestingly, they also identified another ancient
vertebrate gene copy, which they named PDE6I, and has been lost in amniotes.
Anecdotal data on an increasing number of
genes support this view: Drosophila has one
copy each of the Ras, Raf and Notch
genes, as well as of the
genes of the Hox cluster, while
vertebrates have three or more of each of these
genes.