Dermatologists see many patients with disorders that have an important but variable genetic component. Conditions caused by multifactorial inheritance, which account for many patients seen in dermatologic practice, have a strong environmental component in their cause and are generally the most responsive to therapy. Treatment is also possible for many conditions of dermatologic importance that are caused by mutant genes of large effect. At this time, there is little chance of treating the patient in the sense of genetic engineering, but some understanding of the theoretic basis for laboratory manipulation of DNA is a prerequisite to understanding the possible potential of new forms of therapy.