A rare, inherited disorder marked by a small head, height that is shorter than normal, unusual facial features, and other growth and developmental problems. People with Nijmegen breakage syndrome have a weakened immune system, which may cause them to have frequent sinus, lung, and other respiratory tract infections. They are also at an increased risk of developing certain types of cancer, especially non-Hodgkin lymphoma, rhabdomyosarcoma (a type of soft tissue tumor), and brain cancer. Nijmegen breakage syndrome is caused by a mutation (change) in a gene called NBN, which is involved in DNA repair. It is a type of DNA repair deficiency syndrome and a type of autosomal recessive genetic disease.