Huntington disease Achondroplasia Phenylketonuria Delivery Systems Viruses Murine retrovirus - RNA is reverse transcribed in cells and resultant DNA integrated into cell genome.
Genes are gag, pol and env.
Requires two viral constructs. One with pol, packaging sequences, and gene of interest - packaging capable but unable to synthesize structural proteins. Second with structural protein genes but lacking packaging sequences - a helper virus. Combination will package gene of interest in infective particle which is capable of a single cycle of infection only - gene of interest is packaged in proteins produced by the helper virus. Expression efficiency may be lower due to integration site effects.