Therefore, this proliferative response, also known as lymphopenia-induced homeostatic proliferation (LIP), is considered as a common phenomenon for the most polyclonal and even monoclonal naïve T cell repertoire adoptively transferred into a recipient animal under various lympho-depletion settings either genetically (e.g., RAG−/−, TCRβ−/−, CD3ε−/−, and SCID mice) or conditionally (e.g., mice treated with irradiation and cytotoxic agents (3–7). Here, CD3E is linked to lymphopenia.