Inherited monogenic diseases permeate medicine. At the dawn of the genome era, our understanding of the contribution of inherited genetic differences to disease susceptibility is only increasing. Sickle cell disease, the first monogenic disease for which the amino acid and nucleic acid mutations were identified, cautions that knowing the genetic cause of a disease does not easily lead to therapies. There are several potential approaches to treating genetic diseases at the genome level. A particularly intriguing approach is to cure such diseases by correcting the mutation that causes the disease. For sickle cell disease this would entail converting the mutated thymine back to an adenine in codon 6 of the p-globin gene in hematopoietic stem cells and then retrieving the corrected stem cells to the patient as in an autologous stem cell transplant. In the last several years, two major advances have made the possibility of gene correction by gene targeting more promising. The first is the discovery that a DMA double-strand break in the target gene can stimulate gene targeting. We have found that the stimulation can be up to 50,000 fold. The second is our discovery that model zinc finger nucleases can stimulate gene targeting by creating double-strand breaks in the mammalian genome and our preliminary results demonstrating that zinc finger nucleases can be designed to stimulate gene targeting at endogenous sequences. The next step in the study of double-strand break mediated gene targeting is to study the process in primary cells rather than cell lines. This proposal aims to study the process of gene targeting and the use of zinc finger nucleases in hematopoietic progenitor cells. The goal of these studies is both to understand the biology of gene targeting in these cells and to use that understanding to develop targeting as a therapeutic tool to treat monogenic diseases such as sickle cell disease.