IL2RG and Immunodeficiency: A higher tolerance to toxicity is offered by mice that combine Il2rg and recombinase-activating gene (Rag)-1 or -2 knockouts (e.g., BALB Rag−/−;Il2rg−/− or NOD Rag−/−;Il2rg−/−), which generally have the same level of immune deficiency as NSG mice [28,29].