From Jackson Labs: These mice are most commonly called NRG and are extremely immunodeficient. The mice carry two mutations on the NOD/ShiLtJ genetic background; a targeted knockout mutation in recombination activating gene 1 (Rag1) and a complete null allele of the IL2 receptor common gamma chain (IL2rgnull). The Rag1null mutation renders the mice B and T cell deficient and the IL2rgnull mutation prevents cytokine signaling through multiple receptors, leading to a deficiency in functional NK cells. The severe immunodeficiency allows the mice to be humanized by engraftment of human CD34+ hematopoietic stem cells (HSC) and patient derived xenografts (PDX) at high efficiency. The immunodeficient NRG mice are more resistant to irradiation and genotoxic drugs than mice with a scid mutation in the DNA repair enzyme Prkdc.