The X-linked lymphoproliferative disease (XLP), one of six described X-linked immunodeficiencies, stems from a mutation at Xq25 which renders males impotent to mount an effective immune response to the ubiquitous EBV. Purtilo, who first observed this disease in 1969, established a Registry in 1980 to serve as a worldwide resource for the diagnosis, treatment, and research of this condition. Since Purtilo's death in late 1992, the Registry and research unit have not only continued to function as a worldwide consultative service, but have contributed the following. First, the number of affected boys has continued to grow; some 272 among 80 kindreds have been identified. Second, some boys (10%) who inherit the mutated XLP gene are immunologically abnormal even before evidence of EBV exposure. Third, the search for the XLP gene has been narrowed to a small region on Xq25. Its identification is near at hand; once cloned, this gene may well illustrate how the body orchestrates the complex immune response to EBV. Therein lies the justification for the quest for this gene, not only for the benefit of the few surviving boys and those to be born to female carriers, but also for defining its role in defending the body against a ubiquitous DNA virus.