Present efforts at therapy in the major groups of genetic diseases have met with varying degrees of success. In the common multifactorial disorders, in which the genetic predisposition frequently depends heavily on precipitating environmental factors for its expression, treatment is often effective, as in the therapy of diabetes mellitus with insulin or drugs. The chromosome disorders include both those involving the sex chromosomes, such as the Klinefelter and Turner syndromes, in which the administration of the deficient sex hormones is often helpful, and those involving the autosomal chromosomes, such as Down's syndrome, in which specific therapy is at present lacking. . . .