Genetic engineering, more formally known as recombinant DNA technology, allows scientists to pluck genes (segments of DNA) from one type of organism and combine them with genes of a second organism. In this way, relatively simple organisms such as bacteria or yeast, or even mammalian cells in culture and mammals such as goats and sheep, can be induced to make quantities of human proteins, including hormones such as insulin as well as lymphokines and monokines. Microorganisms can also be made to manufacture proteins from infectious agents such as the hepatitis virus or the AIDS virus, for use in vaccines.

Another facet of recombinant DNA technology involves gene therapy: replacing defective or missing genes with normal genes. The first approved gene therapy trials involved children with severe combined immunodeficiency disease, or SCID (Immunodeficiency Diseases), which is caused by lack of an enzyme due to a single abnormal gene. The missing gene is introduced into a harmless virus, then mixed with progenitor cells from the patient's bone marrow. When the virus splices its genes into those of the bone marrow cells, it simultaneously inserts the gene for the missing enzyme. Injected back into the patient, the treated marrow cells produce the missing enzyme and revitalize the immune defenses. Researchers are also investigating the use of gene therapy for such diverse conditions as hemophilia, Parkinson's disease, diabetes, a hereditary form of dangerously high cholesterol, and AIDS.