Molecular Genetic Medicine, Volume II, summarizes progress in several of the most important areas of modern molecular genetics and medicine. The chapters deal with ancient and common genetic diseases, a new infectious disease that threatens to become a world-wide scourge for all of humanity, and two of the most important and still poorly understood causes of mental retardation. The common thread winding through these separate stories is the astounding illumination of all these disorders by modern molecular genetic studies. The book opens with a chapter on the history of the molecular approach to the thalassemias, among the most common and severe of all human genetic diseases. Separate chapters follow covering the history and current state of the fragile X syndrome; the mechanisms of hepatitis B viral gene expression, its relation to liver cancer, and its prevention; and molecular genetics of Down syndrome. Subsequent chapters deal with mammalian X chromosome inactivation; the use of the human hprt locus as a model system for analyzing mutation in human cells in vivo; and the regulatory genes and factors that govern virus replication of HIV-1.