In this study, a T7 plasmid expression vector containing the cDNA of a variant human GST-π gene, hGSTP1*C, was used to examine the translational inhibition of the GST-π mRNA with antisense deoxyribonucleotides (AS-ONs), and to investigate the dependency of the inhibition on ribonuclease (RNAse) H, AS-ON and target mRNA sequence specificity and AS-ON backbone modification. Translational inhibition of hGSTP1*C mRNA showed significant AS-ON concentration-dependency and was both target mRNA and AS-ON sequence specific. Fully modified phosphoromonothioate AS-ONs were less inhibitory than their partial phosphoromonothioate analogs; unmodified AS-ONs were inactive. RNAse H enhanced the translational inhibition by AS-ON specific to the translation initiation region mRNA, and was associated with cleavage of the target mRNA at the site of AS-ON:mRNA hybridization. AS-ONs directed to the A→G and C→T transitions, unique to hGSTP1*C, were more RNAse H-dependent than AS-ONs directed against the translation initiation site, indicating a greater involvement of RNAse H-dependent mRNA cleavage in the mechanism of translational inhibition by AS-ON at the polymorphic site. These data suggest that AS-ONs provide a potentially effective means of specific down-regulation of the human GST-π gene, and demonstrate that the sites of GST-π gene allelo-polymorphism can be targeted to translationally down-regulate the different GST-π gene variants, specifically and differentially targeted. Copyright (C) 1998 Elsevier Science Ireland Ltd.