Apoptosis via CD95 and its ligand is an important mechanism that prevents uncontrolled proliferation of activated lymphocytes and regulates lymphocyte homeostasis. The apoptosis receptor CD95 is a transmembrane protein with an intracellular domain well conserved between CD95 and tumor necrosis factor receptor I, another apoptosis-inducing protein. Because of its functional importance, this domain was designated the death domain. We describe the molecular analysis of the CD95 death domain in a family with autoimmune lymphoproliferative syndrome (Canale-Smith syndrome), T-cell lymphoma, and Hodgkin's disease. A functional defect in apoptosis was detected in cells from the index patient, a 5-year-old girl suffering from Canale-Smith syndrome and a T-cell lymphoma, as well as in her father, who had a history of splenomegaly and mild hemolysis, and her paternal uncle who had been cured of Hodgkin's disease (HD). Expansion of double-negative T cells (CD4-CD8-) was only seen in the index patient. All family members with a functional defect in apoptosis were heterozygous for a point mutation in the death domain of CD95 (A1009G, E256G). We conclude that, within the same family, a defect in apoptosis due to a mutation in the CD95 death domain can be associated with diverse clinical phenotypes, including mild, reversible symptoms and different malignancies.