Viruses are often considered as fragments of cellular RNA or DNA that escaped a long time ago from cellular chromosomes and that evolved later on by capturing additional genes from the genomes of their hosts. However, this view has now been challenged by the discovery of surprising homology between viruses with very distantly related hosts, and by phylogenetic analyses suggesting that genes might also have flown from viruses to cells. We present here phylogenetic analyses of four proteins involved in DNA replication and synthesis of DNA precursors (DNA polymerases delta, ribonucleotide reductases, thymidylate synthases and replicative helicases) and we discuss the reciprocal roles of cells and viruses during the evolutionary history of these enzymes. These analyses revealed numerous lateral gene transfer events between cells and viruses, in both directions. We suggest that lateral gene transfers from viruses to cells and nonorthologous gene replacements of cellular genes by viral ones are an important source of "genetic novelties" in the evolution of cellular lineages. Thus, viruses have definitively to be considered as major players in the evolution of cellular genomes.