Gene therapy is based on the concept that genetic disorders and acquired diseases can be treated by replacing abnormal or absent genes or by modifying their functions. Inherited disorders such as cystic fibrosis and hemophilia, as well as catastrophic diseases such as cancer and AIDS, are prospective candidates for gene therapy. Although cures for these ailments would be welcome, some medical researchers suggest that the range of diseases that can be treated with gene therapy may be limited. According to research scientist Lynn Elwell, "Only a handful of the many diseases that have a genetic basis are amenable to treatment via gene therapy: Genetic disorders caused by single genes." She also adds that "chromosomal disorders, such as Down syndrome, cannot be cured by gene therapy, nor can disorders resulting from complex interactions between many genes or between genes and environmental factors." Advocates of gene therapy contend that this...