Bloom syndrome (BSx) is an autosomal recessive disorder exhibiting a constellation of clinical features including proportional dwarfing, chronic infections, and a significant predisposition to the development of cancer. cytologically, this disease exhibits a many-fold increase in the occurrence of spontaneous chromosome breakage and sister chromatid exchanges (SCEs), the latter being diagnostic. Subtle biochemical alterations have been detected in BSx cells, but reflect consequences secondary to the elusive primary defect. Genetic analyses clearly demonstrate that BSx is a single gene defect. Linkage mapping has been compromised by rare occurrence, by poor family collections, and by a lack of availability of those limited materials which have been collected. Microcell-mediated chromosome transfer (MMCT) offers an alternative mapping approach in which individual normal human chromosomes are transferred from somatic cell hybrids into defective cells to demonstrate phenotypic complementation. Correction of the elevated SCE and chromosome breakage phenotypes of BSx cells coincides with introduction of human chromosome 15. Somatic cell hybrids possessing deletions of chromosome 15 have refined MMCT mapping to l5q23-q26. These mapping data are supported by recent genetic linkage results of others which map the BSx disease to l5q26.l. This application proposes the cloning and characterization of the BSx gene. Genomic clones (cosmids and YACs) representing this 15q26.1 region will be identified and assessed for their ability to complement BSx cells. Concurrently, several innovative approaches (focusing principally on MMCT of additional chromosome IS deletions) will be used to confirm and further refine mapping of the BSx locus. Identification of the BSx complementing genomic clone will permit isolation of a cDNA. Comparisons of mRNA levels and cDNA sequencing data from normal and BSx alleles should confirm this locus to be the primary defect for this disorder. Characterization of this gene and its protein should provide insights into mechanisms which influence recombination, SCE formation, chromosome breakage, and predilection to neoplasia.