In the post-genomic era, the expression and investigation of human (auto)immunity genes seems more relevant than ever. The generation of humanized animal models of human diseases will be useful to study the interplay between genetic and non-genetic factors in disease development and may form a basis for the development of new drugs that act more specifically than the ones currently in use. Transgenic mice have been generated that express various human proteins--candidate autoantigens, disease-associated MHC class II molecules, TCRs and/or CD4--in order to study diseases such as rheumatoid arthritis, multiple sclerosis and diabetes.