Athymic nude mice develop CD4⁺/CD8⁺ T-ALLs when infused with ICN1-expressing cells. Survival curves are shown for cohorts of irradiated nude mice infused with noncultured ICN1⁺ bone marrow–derived cells (A; 5 mice/group; P = .03 by log-rank test), nonirradiated nude mice infused with ICN1⁺ bone marrow–derived cells after 12 days of culture (B; 9 Arf^+/+, 8 Arf^−/− recipient mice, P = .002), and nonirradiated nude mice infused with cultured ICN1⁺ thymus-derived cells (C; 3 mice/group, P = .02). Representative immunophenotypes of T-ALLs arising in each experiment are depicted in panels to the right of the corresponding survival curves (D-F). Although all leukemias had a dominant CD4⁺/CD8⁺ population, several also had CD8 single-positive subpopulations.