NBS1 is tumor supressor gene and in slavic population are two mutations in exon 6 (657del5 and R215W) common. Increased cancer risk in adult heterozygotes was repeatedly reported. Hyperradiosensitivity of NBS carriers should be respected for prevention of malignant tumors. Prevention of secondary tumor was motivation for the detection NBS heterozygotes in children patients with tumors. Authors have found among 688 patients of pediatric oncology 4 heterozygotes of mutation 657del5 and 3 carries of R215W mutation. The difference between these findings and populations data (occurrence 657del5 in newborn 1:165) is not significant, the difference in occurrence R215W mutation in our patients and in population is significant (occurrence R215W in newborn 1:234) and more data is necessary for knowledge of role NBS1 mutation in cancerogenesis in childhood. Oncological control system wasmodified in all 7 detectedNBS heterozygotes andXray investigation were replaces with another method without ionising radiation.

        Key words: tumor supressor gene NBS1, slavic mutations 657del5 and R215W, NBS heterozygotes among children patients with malignancies