The development of innovative molecular techniques such as pulse-field gel electro phoresis, cDNA subtraction libraries and chromosome hopping libraries coupled with the increasing popularity in the prospect of sequencing mammalian genomes, has triggered a resurgence of interest in finding and characterizing genes that playa role in modifying immune processes and diseases. Genetically defined strains of mice (e. g. , inbred strains and recently derived stocks of wild mice) provide ideal models for examining the genetic control of diseases as a result of their syntenic relationship with man in genetic composition as well as linkage conserva tion. Due to the relative ease of producing a specific genotype via appropriate breeding schedules, murine models may provide the only hope for unravelling those complex disease processes under mUltigenic control. This issue of CTMI is a collection of papers on the characterization and mapping of genes involved in mutations and dysregulated immune responses which produce disease phenotypes. These papers were presented at a workshop which was devoted to examining reverse genetic approaches at localizing, cloning and characterizing genes involved in a variety of developmental, autoimmune, neoplastic and infectious disease processes. In the first of three sections, a series of papers outline the most currently used methods of mapping and isolating genes whose products are unknown. The papers, following, are devoted to specific gene systems whose dysregulation is likely to produce mutant or disease phenotypes.