X-linked Lymphoproliferative Syndrome (XLP), or Duncan disease, is an extremely rare genetic disorder of the immune system that causes a defective immune system response to some viral infections, in particular the Epstein-Barr virus (EBV). Defective immune response to those viral infections can be either underactive response (immunodeficiency), or overactive response that leads to many complications. EBV is a herpes virus that causes infectious mononucleosis (IM) and normally it has no long-lasting effects. However, patients with XLP cannot properly attack the virus and that will result in severe, life-threatening IM; glandular fever (in 1/3 of the patients); hypogammaglobulinemia with increased susceptibility to various infections (in 1/3 of the patients); lymphomas especially B-cell lymphoma (in 1/3 of the patients); and/or other abnormalities that vary from one patient to another. Rarely, severe anemia, thrombocytopenia, and inflammation of the blood vessels may also present in those patients. Onset of symptoms ranges from six months to 10 years of age.

There are only 100 families all over the world suffering from XLP, however, unrecognized cases do definitely exist in many countries. Diagnosis relies mainly on blood test that identifies mutations in the corresponding gene or detects the defects in the gene-produced protein known as SAP (Signaling lymphocytic activation molecule - Associated Protein). Supportive treatment is recommended by antibiotics, that prevent other infections, and by immunoglobulin supplements, however, bone marrow transplantation remains the best choice of treatment. About 70% of untreated cases die by the age of 10 years.

Since XLP is transmitted as an X-linked recessive genetic trait, the disorder is usually fully expressed in males only. In most families with XLP, mutations in the SH2 domain protein 1A (SH2D1A) gene have been found to cause the disease. Normally, SH2D1A gene is encoding SAP protein that functions as a regulator of the signal transduction pathways. On the other hand, the causative gene cannot be identified in some families with XLP and research is ongoing to find out which gene is responsible in these families.