RAG1 and lymphopenia: T cell repopulation in response to lymphopenia revealed the existence of two types of lymphopenic environments: severe lymphopenia typical of immunodeficient mice (nude mice, SCID mice, Rag1 KO mice, or lethally irradiated mice) and mild lymphopenia that occurs in normal antibody-mediated T cell depleted mice or in sub-lethally irradiated mice [28,29,31,32].