When two or more viruses, each containing lethal genomic damage, infect the same host cell, the virus genomes can often pair with each other and undergo homologous recombinational repair to produce viable progeny. This process, known as multiplicity reactivation, has been studied in several bacteriophages, including phage T4. Enzymes employed in recombinational repair in phage T4 are functionally homologous to enzymes employed in bacterial and eukaryotic recombinational repair. In particular, with regard to a gene necessary for the strand exchange reaction, a key step in homologous recombinational repair, there is functional homology from viruses to humans (i. e. "uvsX" in phage T4; "recA" in E. coli and other bacteria, and "rad51" and "dmc1" in yeast and other eukaryotes, including humans). Multiplicity reactivation has also been demonstrated in numerous pathogenic viruses.