The identification of the genes which determine biological phenomena, and the study of the control they exert on these phenomena, has proven to be the most successful approach to a detailed understanding of their mechanism. The greatest advances in molecular biology have relied upon the application of the methodology of genetics to the elucidation of the fundamental processes of life at the cellular level. The same statement may be made concerning our understanding of im- munological phenomena. The genetic approach has again proven extremely productive and has permitted us to identify many...