Notably, these subgroups are mutually exclusive to each other and reflect the arrest of T-cell differentiation at different stages, including (a) early blockage at the CD4−CD8− double-negative (DN) stage of thymocyte development for the LMO2/LYL1 group, (b) early cortical T-ALL (CD1a+, CD4+, and CD8+) with expression of TLX1/3 or NKX2-1, and (c) late cortical T-ALL (CD3+, CD4+, and CD8+) with expression of TAL1 (26, 43). The gene discussed is TAL1; the disease is acute lymphoblastic leukemia.