Interestingly, the development of T-ALL involves an early phase characterized by the rise of an abnormal, non-tumorigenic polyclonal CD4+CD8+ double-positive (DP) subset at 2–3 weeks of transplantation and a late phase characterized by the rise in a highly tumorigenic, monoclonal DP leukemic population at 6–10 weeks after transplantation. The gene discussed is CD8A; the disease is acute lymphoblastic leukemia.