The introduction of mutations in key genes involved in the repair of double‐strand gaps in V(D)J recombination, such as Prkdc, Rag1/2, Dclre1c, and Ligase IV, using embryonic stem cell targeting techniques, can also result in severe combined immunodeficiency SCID in mice with impaired T and B cell function.39, 40, 41, 42. The gene discussed is RAG1; the disease is severe combined immunodeficiency.