Most diseases are multifactoral. Transgenic technology permits gene(s) of interest to be expressed in a small manipulatable laboratory animal model. By this process, murine models of human infections can be developed and studied; effects of cytokines in vivo, focally expressed in unique cells can be established and manipulated, and a variety of autoimmune disorders, mimicking human disease can be constructed. In this volume, these approaches for study of human immunodeficiency virus, hepatitis virus, viruses causing tumors and chronic degenerative disorders are described. Also included are chapters of transgenic models of autoimmune disorders like diabetes, systemic lupus and ankylosing spondylitis.