A HeLa cell line stably expressing the human beta-globin gene carrying thalassemic mutations beta(E)/IVS1-6 served as a thalassemia model for repair of aberrant splicing of beta(E)-globin pre-mRNA with antisense oligonucleotides. Treatment of beta(E)/IVS1-6 HeLa cells with a morpholino oligonucleotide targeted immediately upstream of the aberrant 5' splice site activated by the mutations resulted in an increase in the amount of correctly spliced beta(E)-globin mRNA in a dose-dependent and sequence-specific fashion. The repaired beta(E)-globin mRNA was stable and could be translated into full-length beta(E)-globin polypeptide. Application of the same oligonucleotide to erythroid progenitor cells from two beta-thalassemia/HbE patients resulted in an approximately 70% increase in correct beta(E)-globin mRNA and 36% increase in hemoglobin E. The erythroid progenitor cells represent the actual targets for the clinical application of antisense repair of defective pre-mRNAs.