Since 1960, persons with the very rare disorder Bloom's syndrome (BS) have been followed clinically, documenting clinical matters as obtained from their doctors. This has been a worldwide search for cases, though a few in the New York City area are seen (personally, by us) perhaps once every 2-3 years. BS is a rare genetically-determined disorder described in NYC in 1954. The clinical courses of the 169 persons diagnosed BS by 1991 are followed in a program referred to as the Bloom's Syndrome Registry. BS is the prototype of the "chromosome-breakage syndromes." BS cells mutate at a greater rate than any other, and the consequence is the greatest known predisposition to cancers of the types that affect the general human population. We are defining the clinical syndrome and at the same time are studying cells from affected families in the experimental laboratory. BS is a model for learning about cancer. Our contact with families lets us know of cancers arising, but blood, and sometimes tiny biopsies of skin, is taken if available so that (a) the chromosomes can be studied and (b) the gene mutations can be defined in molecular terms.