RAG1 and neoplasm: However, the intrinsic immune system activity (including high level of NK cell activity) of C.B17-SCID and Rag1/2-deficient mice reduced the implantation rate of human HSCs and tumor cells; NK cells from C.B17-SCID and Rag1/2-deficient mice show cytotoxicity toward human HSCs and tumor-initiating cells (25),further limiting their application in tumor models.