Hypomorphic mutations in NBS1 (officially known as NBN) cause the Nijmegen breakage syndrome (NBS-OMIM: 251260), an autosomal-recessive disorder characterized by progressive microcephaly, mild to moderate growth retardation, dysmorphic facial features, immunodeficiency, and predisposition to cancer, particularly to lymphoid malignancies. The gene discussed is NBN; the disease is immune system disorder.