Stem cells are unique among cell types found in the human body: These cells are pluripotent; that is, they can develop into any of the more than 200 cell types in the human body. A major goal of stem cell research is to develop treatments for patients who suffer from devastating and currently incurable conditions such as AIDS, Alzheimer’s, liver disease, diabetes, Parkinson’s disease, muscular dystrophies, spinal cord injuries, and inborn errors of metabolism. These patients might be treated with gene-modified or gene-corrected patient-specific human embryonic stem cells (hESCs). In the hESCs used for treatment, the bad or defective gene must be either replaced or repaired with a good or effective gene. In some cases, it may be important that the patient’s hESCs be provided with a disease-fighting gene. Here, the genes need to be placed in safe sites in the genome. For example, we might be able to treat AIDS patients using hESCs modified to contain a gene to make them resistant to the HIV-1 virus or patients with Alzheimer’s disease might be treated with neural stem cells equipped with a new gene that fights the development of Alzheimer plaques throughout the brain. Unfortunately, the current state-of-the-art in gene delivery does not allow scientists to insert genes safely at any given site in the genome. We also lack efficient techniques to readily repair defective genes by exchanging them with good genes. Such technologies will be key to realizing the full potential of embryonic stem cell therapy. In this reporting period we have begun to develop zinc finger recombinase enzymes as tools that will allow us to achieve these goals. We have built special proteins called zinc finger proteins to bind certain target sites in DNA of the human genome and we have developed a new method that is allowing us to fine-tune zinc finger recombinases to target these important sites. As proof of principle of our ability to program zinc finger recombinases to target new sites, we have been able to demonstrate that we can radically reprogram the specificity of these enzymes. In the next reporting period, we will combine these advances to actually modify our target genes in hESC lines. We anticipate that this technology will be highly efficient and allow any individual patient’s ESCs to be corrected at the genetic level.