The X-linked lymphoproliferative syndrome is triggered by Epstein-Barr virus infection and results in fatal mononucleosis, immunodeficiency, and lymphoproliferative disorders. This study shows that the mutation responsible for X-linked lymphoproliferative syndrome is genetically linked to a restriction fragment length polymorphism detected with the DXS42 probe (from Xq24-q27). The most likely recombination frequency between the loci is 4%, and the associated logarithm of the odds is 5.26. Haplotype analysis using flanking restriction fragment length polymorphism markers indicates that the locus for X-linked lymphoproliferative syndrome is distal to probe DXS42 but proximal to probe DXS99 (from Xq26-q27). It is now possible to predict which members of a family with X-linked lymphoproliferative syndrome are carrier females and to diagnose the syndrome prenatally.Epstein-Barr virus (EBV)-associated diseases-infectious mononucleosis, nasopharyngeal carcinoma, and Burkitt lymphoma-cause considerable morbidity and mortality. After EBV infection B lymphocytes are efficiently transformed into persistently proliferating lymphoblastoid cells. These cells can be killed by other proliferating cells that are evoked in an immune response. Infection of children is usually asymptomatic, but nonimmune adults develop infectious mononucleosis. EBV-induced lymphoproliferation is fatal in some new world primate species and in a small number of human subjects with both inherited and acquired immunodeficiencies. One such defect results from a mutation of the X chromosome.This X-linked lymphoproliferative (XLP) syndrome mutation in some way prevents the immune system of an affected male from making an appropriate response to EBV infection (1-4). Before EBV infection occurs, males with an XLP mutation have normal cellular and humoral immune responses (5) and respond normally to bacterial and viral infections other than EBV. After EBV infection, about 75% of males with XLP develop fatal infectious mononucleosis, with liver destruction often the immediate cause of death. Boys with XLP who survive EBV infection have defects in humoral and cellular immunity and a high incidence of lymphoma (4-6). The interaction between EBV and B lymphocytes appears normal in affected males, but EBV-induced activation of anomalous killer T cells may cause the lymphoreticular cytopathology that leads to symptoms (5).Restriction fragment length polymorphism (RFLP) markers are being used to map genetic disease loci and to construct the human genetic map (7). In this report we describe the use of X chromosome RFLPs to study a family with XLP. The discovery of RFLPs flanking the XLP locus will enable diagnosis prior to infection and may prove an important step toward understanding XLP at the molecular level. [a-32P]dATP. The blots were washed in 0.08% NaCl/0.1% NaDodSO4/0.08% Tris base/ 0.02% H3PO4 at 60°C and autoradiographed with an intensifying screen at -70°C.RFLP probes used and their locations are as follows: