Our goal is to evaluate the future potential of targeted gene modification with a view to its eventual use for the gene therapy of beta-thalassemia and sickle cell disease. Because targeted gene modification is such a novel approach to the genetic manipulation of mammalian genomes, much of the planned work will be directed towards fundamental aspects of the procedure. The specific objectives are to determine (i) what types of targeted modification of the human beta-globin gene can be achieved, (ii) what factors influence the efficiency of the procedure, and (iii) what conditions must be met to obtain correctly regulated and full level expression of a modified beta-globin gene. The experiments related to the types and efficiency of targeting will be primarily with cells in culture. The experiments related to expression will first be carried out in tissue culture. Later experiments will involve normal and thalassemice mice having their bone marrows repopulated by stem cells with a beta-globin locus modified by gene targeting. At the conclusion of the study it is expected that a plasmid will have been constructed and tested which is capable of introducing a gene into the beta-globin locus of hematopoietic stem cells from beta-thalassemia or sickle cell homozygotes in such a way that the resulting red cells function adequately, and that a comparable plasmid will have been shown to effectively cure thalassemic mice without having to use life-threatening techniques.