With present technology, nearly any DNA virus can be employed as an expression vector. The characteristics of such vectors are diverse and depend on the intrinsic biological properties of the parental virus. With some vectors, construction of recombinant viruses is straightforward and expression is routinely obtained. Thus far, the greatest uses of virus vectors have been for determination of the targets of humoral and cell mediated immunity, experimental animal protection studies, protein production in cultured cells, and analysis of protein function and processing. Some recombinant virus vectors are being tested as candidate live vaccines for medical and veterinary purposes; the products of others are being evaluated as subunit vaccines.