Tumor watch. (A) The indicated cohorts of littermate-matched mice were prospectively established in a tumor watch. Mice were followed for 18 months and moribund animals were killed. Leukemia occurred in mCG-PR^+/− mice, but not in littermate controls. There was no significant difference in latency or penetrance in mCG-PR^+/− × Rara^+/− vs mCG-PR^+/− × Rara^+/+ mice. (B) Maximum diameter of largest cervical lymph node at the time of killing. (C) Spleen size in the indicated mice at the time of killing or at 18 months of age, whichever came first. (D) Peripheral WBC counts at the time of killing or at 18 months of age. (E) Mice shown in Figure 3 were followed, and moribund mice were killed. There was no significant difference in survival by genotype of transplanted bone marrow cells.