For gene therapy to realize its clinical potential, there must be efficient and safe strategies of delivering therapeutic genes to somatic cells in vivo. Perhaps this problem simply represents a special case of drug delivery in which the therapeutic gene constitutes the drug. But the relevance of traditional drug development to technological developments in gene therapy is questionable. Recombinant genes that can independently express a therapeutic RNA or protein are extraordinarily large and polar molecules — 10,000 times larger than traditional pharmaceutical agents. Moreover, the administration of genes as therapy requires vehicles —so-called vectors — that encapsulate the gene and . . .