Although IkL/L mice die from T-cell acute lymphoblastic lymphomas/leukemias (ALL) at 4–6 months of age, the animals used here showed no signs of transformation in the thymus, as defined by CD4 and CD8 profiling, TCR Vα and Vβ chain usage, and the absence of intracellular Notch1 in DP thymocytes14, 15. The gene discussed is NOTCH1; the disease is acute lymphoblastic leukemia.