Nijmegen breakage syndrome (NBS), is a rare autosomal recessive congenital disorder causing chromosomal instability, probably as a result of a defect in the double Holliday junction DNA repair mechanism and/or the synthesis dependent strand annealing mechanism for repairing double strand breaks in DNA (see Homologous recombination). The name derives from the Dutch city Nijmegen, where the condition was first described. Most people with NBS have West Slavic origins. The largest number of them live in Poland.