The ability of antisense oligonucleotides to induce frameshifting in cultured mammalian cells was examined by co-transfection of CV-1 cells with p2lucAZ1PKdel and increasing amounts of 2 0 -O-Methyl antisense oligonucleotides AZ1B as described in Materials and Methods. In the absence of antisense oligonucleotide frameshifting levels were determined to be 1.1%, whereas a graded increase in frameshift levels was observed upon the addition of AZ1B ( Figure 6 ). Maximal frameshifting levels were 13% in the presence of 2 mM AZ1B in the transfection media.