Two major solutions have been proposed: one is to use cells with lower immunogenicity to fabricate cell sheets, like mesenchymal stem cells (MSCs), allogenic cells, or autologous cells [36–38]; the other is to use immunodeficiency animal models such as severe combined immunodeficiency (SCID) mice [39], Rag2-Il2rg double knockout mice [40], and nude rats [41]. The gene discussed is RAG2; the disease is severe combined immunodeficiency.