Mutations in the SRV-1 gag-pro frameshifting signal were made in an abridged version of plasmid SF2 (25) which is derivative of pSFCASS5 (26), a frameshift reporter construct. In this version, the entire BglII-NcoI fragment of pSF2 was replaced by a synthetic dsDNA fragment (5 0 -G ATCTTAATACGACTCACTATAGGGCTCATTTAA ACTAGTTGAGGGGCCATATTTCGC-3 0 , a SpeI restriction site is underlined). This yielded plasmid pSF208 in which the original GGGAAAC slippery sequence has been replaced by the more slippery UUUAAAC sequence (26) . pSF208 was digested with SpeI and NcoI, and sets of complementary oligonucleotides corresponding to the various mutants were inserted. A list of oligonucleotides is available upon request. All constructs were verified by automated dideoxy sequencing using chain terminator dyes (LGTC, Leiden).