FAS and sinus histiocytosis with massive lymphadenopathy: One previous study reported a patient with autoimmune lymphoproliferative syndrome developing histiocytic sarcoma in a background of RDD with a germline missense mutation in exon 9 of the TNFRSF6 gene encoding Fas, which elucidated a probable Fas/Fas ligand pathway–mediated pathogenesis [51].