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Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability disorder characterized by a high incidence of pediatric hematologic malignancies. Majority of patients affected are of Slavic origin and share the same founder mutation of 657del5 within the NBN gene encoding protein involved in DNA double-strand breaks (DSB) repair. We report a case of a pediatric patient with NBS, who developed t(9;11)/AF9-MLL-positive AML as a second malignancy after successful treatment of T-NHL. The coexistence of NBN and MLL mutations suggests that the profound dysfunction of NBN may promote alterations of MLL that is mediated by error-prone non-homologous end joining pathway particularly in patients treated with DNA topoisomerase II inhibitors. Pediatr Blood Cancer 2014; 61:1469–1471. © 2014 Wiley Periodicals, Inc.