To the Editor.—
The X-Linked Lymphoproliferative Syndrome Registry was recently established and is funded by the National Cancer Institute. We invite referrals of possible cases of the X-linked lymphoproliferative syndrome as well as unusual complications associated with infectious mononucleosis. The following criteria may be used to ascertain whether a case is a candidate for the registry: (1) phenotypes (N Engl J Med 297:1077-1081, 1977) of the proliferative types including fatal infectious mononucleosis, immunoblastic sarcoma of B cells, immunodeficiency with hyper-IgM following infectious mononucleosis, or American-Burkitt's lymphoma, aproliferative phenotypes such as agranulocytosis, aplastic anemia, or acquired agammaglobulinemia, (2) the phenotypes of X-linked lymphoproliferative syndrome in two or more maternally related males, and (3) in affected males, after infection by Epstein-Barr virus, infected B cells grow spontaneously, saliva transforms cord lymphocytes, and Epstein-Barr virus antibody is usually absent in serums.By studying affected patients and their families, we seek to increase knowledge