RAG2 and T-cell leukemia: To prevent mature T cell differentiation and to assess the ability of STAT5 GOF to induce immature, thymic T cell leukemia, we crossed our hyperactive STAT5 mice and STAT5B WT controls with Rag2-deficient mice, in which T cell development is blocked at the DN3 stage due to a lack of VDJ recombination and surface TCR αβ/γδ chain expressions (45) (Figure 2A).