Eukaryotic cells have developed two independent pathways to repair double-strand breaks in their DNA: non-homologous end-joining (NHEJ) and homologous recombination (HR). A complex containing Mre11, Rad50 and Nbs1 proteins has been implicated in double strand break repair, but its exact function is unclear. In the November 7 Nature, Hiroshi Tauchi and colleagues characterized the role of the Mre11-Rad50-Nbs1 complex in vertebrate cells by creating Nbs1 knockout cells (Nature 2002, 420:93-98).

The recombinogenic chicken B-cell line DT40 was used to generate cells lacking a functional Nbs1 allele. The knockout cell line was hypersensitive to ionizing radiation and displayed abnormal S-phase checkpoint behavior. These cells did not exhibit a profound end-joining defect. But loss of Nbs1 resulted in reduced gene conversion and sister chromatid exchange events. Tauchi et al. used a site-specific enzyme (I-SceI) to create a double-strand break and demonstrated that the HR repair pathway required a...

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