Current therapies for most human genetic diseases are inadequate. In response to the need for effective treatments, modern molecular genetics is providing tools for an unprecedented new approach to disease treatment through an attack directly on mutant genes. Recent results with several target organs and gene transfer techniques have led to broad medical and scientific acceptance of the feasibility of this "gene therapy" concept for disorders of the bone marrow, liver, and central nervous system; some kinds of cancer; and deficiencies of circulating enzymes, hormones, and coagulation factors. The most well-developed models involve alteration of mutant target genes by gene transfer with recombinant pathogenic viruses in order to express new genetic information and to correct disease phenotypes--the conversion of the swords of pathology into the plowshares of therapy.