Gene therapy has been proposed as a definitive cure of beta-thalassemia. We applied a gene targeting approach, based on the introduction of small DNA fragments (SDF) into erythroid progenitor cells, to specifically modify the beta-globin gene sequence at codon 39. The strategy was first tested in normal individuals by delivering mutant SDF that were able to produce the beta-39 (C->T) mutation. Secondly, wild-type SDF were electroporated into target cells of beta-3i9/beta-39 b-thalassemic patients to correct the endogenous mutation. In both cases, gene modification was assayed by allele-specific polymerase chain reaction of DNA and mRNA, by restriction fragment length polymorphism analysis and by direct sequencing.