Phage (viruses of bacteria) are selfish genetic elements, but the reproductive strategy of lysogenic phage is to integrate into host DNA, repress phage gene expression except for the repressor itself, and allow host to grow and multiply. Comparison of lysogenized phage genomes often shows an extra gene inserted, "more DNA." The inserted gene may have its own promoter and terminator, allowing expression independent of normal phage control mechanisms. The whole structure is termed a moron, cargo gene, or lysogeneic conversion gene. It may contribute to host cell fitness, benefiting itself by benefiting the host. If the bacterium is a pathogen, the moron/fitness factor may be a virulence factor.

For bioinfotropes, the signature characteristics of fitness factors in phage are:

  • the phage should by lysogenic, not purely lytic
  • the gene is not important to the phage life cycle
  • the fitness factor gene may have non-phage homologs
  • the gene may appear inserted relative to the comparable gene order of a related phage
  • the gene may have its own promoter and terminator, allowing expression during lysogeny

Genes with these properties are lysogenic conversion genes by definition, but may or may not be fitness factors. Understanding fitness factors may explain features of the relationship between the host cell and its environment.

Fitness factors may remains intact after mobility functions are lost during the domestication of mobile genetic elements.