Four human reteroviruses have been identified. All infect CD4 bearing cells. These were only identified in the 1980's when it became possible to culture T-cells in vitro . The reverse transcriptase is highly error prone and rapid genetic variation is a feature of this group. Spumavirinae - no pathology known Defective virus Virus which has lost a gene that is essential for replication and can, therefore, only undergo productive infection if the cell that is harbouring the virus is superinfected with a helper virus that can supply the function of the lost gene.
Many oncogenic retroviruses are defective.
Defective retroviruses which integrate into the host genome and are passed down from generation to generation. Two percent of the human genome is made up of endogenous retroviral sequences. Life cycle of lentiviruses Oncogenes Viruses of the Oncovirinae subfamily contain an extra gene termed , 'onc'. These genes are called oncogenes because their expression in the virus infected cell is associated with tumour production. These genes were originally acquired from the host cell itself.