The patient was found to have a heterozygous mutation in the Fas cell surface death receptor (FAS) gene (NM_003824:c.517G>A; [p.Glu173Lys]), previously described in patients with autoimmune lymphoproliferative syndrome.12,13 This patient is currently undergoing assessment for allogeneic hematopoietic stem cell transplantation (HSCT). The gene discussed is FAS; the disease is autoimmune lymphoproliferative syndrome.