Herein, we inoculated human α-synuclein preformed fibrils (PFFs) into the brain of rats with X-linked severe combined immunodeficiency (X-SCID) and demonstrated that α-synuclein PFFs got incorporated into SNCA−/− hiPSC-derived mDA neuronal grafts. The gene discussed is SNCA; the disease is severe combined immunodeficiency.