T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological neoplasm with poor outcome for patients with primary resistant or relapsed disease. We have identified T-ALL patients with elevated levels of SOX11 (SOX11 high), a factor that is normally not expressed in normal T-cells, suggesting that SOX11 might have oncogenic potential in T-ALL. Interestingly, SOX11high T-ALL is mostly restricted to theTAL/LMO genetic subgroup and is associated with genetic inactivation of. PTEN. To test if SOX11 is an oncogenic driver of TAL/LMO-based T-ALL, we have developed a novel R26-SOX11 mouse model and will induce conditional overexpression of SOX11 in normal and Pten-null T-cells. In addition, we have also identified a striking correlation between the presence of the recurrent MYCN-stabilizing P44L mutation and high SOX11 expression in TAL-LMO rearranged T-ALLs, suggesting that MYCN might be one of the factors that can drive aberrant SOX11 activation during T-cell transformation. Altogether, we believe that SOX11 serves as a critical mediator of T cell transformation in a subset of TAL/LMO rearranged T-ALL. Additional insights in the role of SOX11 in T-ALL disease biology might eventually lead to the identification of novel druggable targets for the development of less toxic treatments for a subset of human T-ALL patients.