The Nbs1 gene acts on DNA double-strand break repair and DNA damage-induced checkpoint activation. Mutations in this gene are associated with Nijmegen breakage syndrome, an autosomal recessive chromosomal instability syndrome characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. The Nbs1/Mre11/Rad50 (MRN) complex possesses endonuclease activity, generating 3’-overhang ssDNA essentially required for homologous- recombination-repair. Deletion of any member of the MRN complex leads to lethality at cellular and embryonic levels. Strikingly Dr. Yang obtained viable Nbs1 ES cells (Nbs1neo/hyg) via gene targeting by replacing its two exon 6 alleles with neomycin and hygromycin resistance genes, respectively; and its fibroblasts were also derived (Yang et al., EMBO J, 2006). Those cells display normal p53 activation and defective ATM/ATR signalling, but with only a mild reduction of HRR. The availability of the viable Nbs1 ES and MEF cells will enable us to explore various aspects of the DNA damage response, particularly in the DNA DSB repair. In the course of this program, Dr. Danielsen will screen the Nbs1-extragenic-suppressor (NESR) gene required for the remaining HRR in Nbs1 cells, and explore its molecular mechanisms in regulating the HRR and association with tumor malignancy.