Strategy for the construction of the full-length cDNA clone of an attenuated live PRRSV vaccine strain, HuN4-F112. The organization of the viral genome is shown, as are the positions of the unique restriction sites used for cloning purposes. The numbers 1a, 1b, 2a, 2b and 3 through 7 indicate the PRRSV open reading frames. An SP6 RNA promoter with two nontemplated G residues preceded the viral genome. The complete viral genome was divided into six fragments flanked by unique restriction sites, represented by the horizontal lines labeled A through F. The fragments were inserted into the modified pBluescript II SK(+) vector.