Utilizing modern molecular biologic techniques such as gene cloning, microarray hybridization, and gene transfer, my laboratory is involved in the elucidation, diagnosis and ultimately treatment of single-gene defects at the molecular level. Using human arginase deficiency, a defect in the urea cycle, as a model system, we are exploring, in close collaboration with the laboratory of Dr. Stephen Cederbaum, the molecular structure and tissue-specific regulation of the arginase genes in health and disease. We have determined the mutation sites in a large cohort of arginase-deficient patients worldwide, have identified important regulatory sequences governing tissue-specific expression and extinction by transferring the cloned genes and promotor sequences into various recipient cells types, and have constructed two arginase-deficient knockout mouse models. Alternatives for gene replacement therapy have been explored by re-directing the transfected genes to different subcellular compartments. Both the ontogeny and the regulated expression of the arginase isozymes are being studied in a variety of developmental and adult systems and in certain malignant tumors (e.g., prostate cancer) in which their expression is induced. In addition, as director of the UCLA Medical Center’s Diagnostic Molecular Pathology Laboratory, I can also offer my trainees exposure to the latest clinical applications of molecular biology in the diagnosis of a wide range of genetic, neoplastic, and infectious diseases as well as DNA fingerprinting. A major effort in this area has been our pilot project, funded by the Human Genome Initiative, to study the effectiveness of large-scale population screening for cystic fibrosis mutations by DNA analysis. This experience has served as a paradigm for the continuing development of other important DNA-based screening tests in our laboratory.