The goal of the proposed research is to generate a family of novel, highly sequence-specific DNA binding proteins with the ability to modify the structure or expression of specific genes in human cells. The starting point for the generation of the Gene-Specific-Reagents (GSR's) are existing proteins known as homing endonucleases, that can bind and cleave unique, long (15-40 bp) DNA targets or homing sites with high specificity in vivo. The mechanism by which these homing endonucleases bind to their specific targets suggest that it may be possible to generate GSR's that can bind to any target sequence of human cell DNA with relatively high specificity. Homing endonuclease variants with the ability to modify the structure or functions of specific human genes would provide new opportunities to investigate human gene structure and function. The availability of human GSR's may also allow modification of altered human genes responsible for specific diseases or genes of pathogenic microorganisms including bacteria and viruses, parasites, etc.