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Bloom's syndrome (BS) is a rare autosomal recessive disorder characterized by genomic instability and cancer predisposition. The underlying genetic defect is mutation of the BLM gene, producing deficiency in the RecQ helicase BLM (Bloom's syndrome protein). The present article begins by introducing BLM and its binding partners before reviewing its known biochemical activities and its potential roles both as a pro-recombinase and as a suppressor of homologous recombination. Finally, the evidence for an emerging role in mitotic chromosome segregation is examined.

The RecQ family of DNA helicases is highly conserved in evolution from bacteria to humans. Of the five known human RecQ family members, three (BLM, WRN and RECQ4, which cause Bloom's syndrome, Werner's syndrome and Rothmund-Thomson syndrome respectively) are mutated in distinct clinical disorders associated with cancer predisposition and/or premature aging. BLM forms part of a multienzyme complex including topoisomerase IIIalpha, replication protein A and a newly identified factor called BLAP75. Together, these proteins play a role in the resolution of DNA structures that arise during the process of homologous recombination repair. In the absence of BLM, cells show genomic instability and a high incidence of sister-chromatid exchanges. In addition to a DNA structure-specific helicase activity, BLM also catalyses Holliday-junction branch migration and the annealing of complementary single-stranded DNA molecules.