In February 1997 it was reported that pig cells contain a retrovirus capable of infecting human cells (at least, in vitro). This is troublesome because of the efforts that are being made to transplant pig tissue into humans (e.g., fetal pig cells into the brains of patients with Parkinson's disease). Transplant recipients must have their immune systems suppressed if the transplant is to avoid rejection. Could immunosuppressed patients be at risk from the retroviruses present in the transplanted cells? The probability that the original hosts for HIV-1 and HIV-2 were some other primate suggests that retroviruses can move from one species to another.A typical, "minimal" retrovirus consists of:
Reverse transcriptase is a DNA polymerase that uses RNA as its template. Thus it is able to make genetic information flow in the reverse (RNA ->DNA) of its normal direction
(DNA -> RNA).
All the proteins in the virus particle are encoded by its own genes.
When a retrovirus infects a cell
The genome of retroviruses
The retroviral genome also contains a packaging signal sequence ("P") which is needed for the newly-synthesized RNA molecules to be incorporated in fresh virus particles [Example].
Most retroviruses also contain one or more additional genes. Some of these represent RNA copies of genes that earlier were picked up from their eukaryotic host. Several cancers in animals are caused by retroviruses that have, at some earlier time, picked up a proto-oncogene from their mammalian host and converted it into an oncogene.