Most patients are of Slavic ancestry.
Recently we cloned the NBS1 gene which codes for nibrin, a member of the RAD50 protein complex of DNA double-strand break repair and recombination. We identified a total of 7 truncating mutations in different NBS patients, the majority being homozygous for mutation 657del5 in exon 6. We found this mutation in 68 unrelated NBS patients. Based on epidemiological data it has been proposed that NBS heterozygotes also have an elevated incidence of malignancies. In order to determine the population incidence of NBS mutations we now undertook a study to estimate the frequency of the 657del5 mutation in the Czech Republic and Poland.