Extensive preclinical studies using rAAV2 containing several different transgenes in a variety of animal models have shown efficient and persistent gene transfer and expression with minimal toxicity. The parent virus (wild-type AAV2) is a naturally occurring, non-replicating virus that depends on a helper virus, such as adenovirus, for replication. The recombinant AAV2 vector is unable to replicate in target host cells because it lacks the AAV genes, whose protein products are also required in trans, for replication and packaging of progeny virus. Extensive epidemiological studies have found AAV2 to be non-pathogenic.