Analysis of p19^Arf and Arf-Gfp reporter expression in Eμ-Myc transgenic mice. (A) Levels of p19^Arf were determined in splenocytes from control (lane 1) and Eμ-Myc transgenic animals at the designated stages of disease (lanes 2-10). Cultured Arf-null (lane 11) and p53-null B cells (lane 12) served as controls. Whereas splenocytes from mice in the latent phase do not express GFP, significant proportions of splenocytes (more than 20%) expressed the Arf-Gfp allele at later stages of disease. The overall levels of p19^Arf were diminished or absent in 4 of 5 intermediate-phase mice (lanes 4-8) and almost invariably absent in terminal-phase mice (2 examples of 22 mice are shown in lanes 9 and 10). (B) High-level expression of the Arf-Gfp allele was initially detected by FC analysis in a subset of peripheral white blood cells (▪) just prior to the development of lympholeukemia, as manifested by white blood cell counts of more than 30 × 10⁹/L (30 000/μL) (♦). Animals that became moribund were killed approximately 2 weeks later (▴; terminal phase). Error bars indicate SD from the mean. (C) B220⁺ BM and Sp cells taken during the intermediate phase of disease from Eμ-Myc Arf^GFP/GFP mice initially generated a subpopulation of cells that express high levels of the Arf-Gfp allele.