RAG1 and immunodeficiency disease: One reason for choosing 2-cell stage embryos in our experimental design was that IL2RG or RAG1 gene knockout caused severe immunodeficiency in the animals, resulting in a short lifespan18, 20, 24, 27, while gene modification at the 2-cell stage embryo may generate chimeric animals, which can survive and pass on the mutated RAG1 or IL2RG gene to their offspring.