Transplant recipients must have their immune systems suppressed if the transplant is to avoid rejection. Infection of a host cell requires that the cell have a surface protein that can serve as a receptor for the envelope protein of the retrovirus. The envelope protein of HIV-1 binds to CD4 molecules. All the proteins in the virus particle are encoded by its own genes.
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 .