An integrated retrovirus effectively becomes part of the cellular genome, but with the difference that the virus to a large extent retains control over its own expression through nontranslated sequences in the long terminal repeat (L TR). Some retroviruses also code for nonstructural proteins that further regulate proviral expression. Integration changes the cell genome; it adds viral genes, and in the case of transducing retroviruses also adds cell-derived oncogenes that have been incorporated into the viral genome. Integration can also have consequences for cellular genes. The...