In severe combined immunodeficiency (SCID) gene homozygous mutation or recombination activation gene 1 (Rag1) or Rag2 homozygous mutation mice, accompanied by mutation of the interleukin 2 receptor gamma chain (IL2Rg) locus, comparing with previous immunodeficiency mouse models, the implantation and function of human hematopoietic stem cells (HSC) and peripheral blood mononuclear cells (PBMC) are greatly increased (38). This evidence concerns the gene RAG2 and severe combined immunodeficiency.