11 At the protein level the PHF5a sequence is 100% identical in all vertebrates studied (Figure 2 ). If the comparison is limited to multicellular organisms, which includes rat, mouse, human, zebra fish, the fruit fly Drosophila melanogaster, the mosquito Anopheles gambiae, the nematode C. elegans, and the plant Arabidopsis thaliana, the degree of sequence identity is over 80%. Even the most distant PHF5a ortholog found in the budding yeast Saccharomyces cerevisiae is 55% identical to the

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... rate PHF5a. This suggests a very high selective pressure to conserve PHF5a sequences during evolution. Interestingly, as it was suggested by Oltra et al. (2003) that the PHF5a sequence was conserved without even a single amino acid change for a period of more than 530 million years, the date of the oldest fish fossil found. Furthermore, to date no ortholog of PHF5a could be found in the genomes of prokaryotic organisms, therefore, it is possible that its presence and function is restricted to eukaryotic organisms. The level of evolutionary conservation indicates that PHF5a may contribute to basic cellular function (Stanchi et al., 2001) .