A portion of the feline leukaemia virus genome is not endogenous in cat cells.

Although viral sequences closely related to feline leukaemia virus are represented in multiple copies in cellular DNA of all domestic cats, a specific fraction was present only in the virus-infected cells. This fraction was detected by viral cDNA enriched by a prior absorption of a total complementary DNA (cDNA) transcript with normal cat liver DNA. The recycled cDNA hybridized well with the cellular DNA of virus-infected cells, but to a lesser extent with DNA from uninfected cat cells. The probe was used to differentiate virus-positive from virus-negative tumour tissues of cats. The same approach with cDNA of another endogenous feline virus, RD114, failed to show any difference between a virus-infected cell line and normal cells, including both virus-inducible and non-inducible lines.