A virus that by mutation has lost the ability to be replicated in the host cell without the aid of a helper virus. The virus particles (virions) contain all the viral structural components; they can attach, penetrate, and release their nucleic acid (RNA; DNA) within the host cell. However, since the mutation has destroyed an essential function, new virions will not be made unless the cell was simultaneously infected with the helper virus, which can provide the missing function. Only then will the cell produce a mixed population of new helper and defective viruses. Occasionally, when their nucleic acids become integrated in the DNA of the host cell, defective viruses persist in nature by propagation from mother cell to daughter cell. See Animal virus, Mutation
The most important group of defective viruses are deletion mutants. They are derived from their homologous nondefective (wild-type) virus through errors in the nucleic acid replication that result in the deletion of a fragment in the newly synthesized molecules. The defective nucleic acid must be capable of self-replication, at least in the presence of the wild-type virus, and must combine with other viral components to form a particle in order to exit the cell.
The defective RNA tumor viruses are deletion mutants. Mammalian and most avian sarcoma viruses require a nondefective leukemia virus as a helper virus. Usually the specificity for a certain type of host cell exhibited by the defective virion depends on the helper virus, indicating that one of the virion surface proteins has been furnished by the helper virus gene. These proteins are involved in interactions with cellular surface receptors, and thus determine whether a cell can serve as a host for viral infection.