Cytotoxicity of oligonucleotides. The cytotoxicities of antisense oligonucleotides were determined after treatment of NIH 3T3 MDR cells with various concentrations of antisense/Lipofectamine 2000 complexes and further incubating them for 64 h as described. Closed circles, control cells treated with Lipofectamine 2000 only; closed squares, HNA (GS1954); closed triangles, HNA gapmer (GS1956); open squares, phosphorothioate (5995); open triangles, 2′-O-ME gapmer (13 158). Values represent cell population in percentage with 100% taken for untreated NIH 3T3 MDR cells. Means and standard errors of 3–6 determinations.