Nijmegen breakage syndrome caused by NBS1 germline mutations is a rare autosomal recessive disease with clinical features that include microcephaly, increased radiosensitivity, and predisposition to cancer. NBS1 plays a key role in DNA double-strand break repair and the maintenance of genomic stability. We screened 87 glioblastomas for NBS1 mutations (all 16 exons). Single-strand conformation polymorphism followed by direct DNA sequencing revealed 12 NBS1 mutations (8 missense and 4 intronic mutations) in 9 (32%) of 28 primary (de novo) glioblastomas carrying 2 or more TP53 mutations. None of the NBS1 mutations has been previously reported as a germline mutation in Nijmegen breakage syndrome patients. NBS1 mutations were not detected in 19 primary glioblastomas with 1 TP53 mutation or in 21 primary glioblastomas without TP53 mutations. Secondary glioblastomas that developed through progression from low-grade or anaplastic astrocytoma had TP53 mutations in 16 (84%) of 19 cases, but none contained mutations of the NBS1 gene. These results suggest that multiple TP53 mutations in glioblastomas are due to deficient repair of DNA double-strand breaks caused by mutational inactivation of the NBS1 gene.