Mutations in the NBS1 encoding gene, NBN, are associated with Nijmegen breakage syndrome (NBS) (OMIM# 251260) characterized by microcephaly, immunodeficiency, growth and intellectual disability, radiosensitivity, and cancer predisposition.1, –, 3 Mutations in RAD50 gene RAD50 are related to NBS-like disorder (NBSLD) (OMIM# 613078).4 The clinical features of patients with NBSLD are very similar to those with NBN, including microcephaly and intellectual disability but not infections, immunodeficiency, or cancer predisposition. The gene discussed is NBN; the disease is microcephaly.