T-cell malignancies result from Notch activation after “spontaneous” Arf promoter engagement. (A) Survival of cohorts of Arf^Cre/+; Rosa26^LSL-Notch (n = 10) and Arf^Cre/−; Rosa26^LSL-Notch (n = 23) mice developing T-cell tumors, compared with tumor-free Arf^+/−; Rosa26^LSL-Notch littermates (n = 24). The differences in median survival of the Arf^Cre/+; Rosa26^LSL-Notch mice (180 days) versus Arf^Cre/−; Rosa26^LSL-Notch mice (109 days) were statistically significant by log-rank test (P = .0002), indicating that biallelic Arf inactivation accelerates disease onset. (B) Cre-mediated activation of the knock-in Notch allele results in coexpression of IRES-GFP. GFP expression in the hematopoietic and lymphoid tissues of 21 Arf ^Cre/−; Rosa26^LSL-Notch mice killed after developing overt clinical disease was assayed by flow cytometry. (C) Spleen cells from Arf^Cre/+; Rosa-^LSL-Notch mice with clinical disease were sorted into GFP⁺ and GFP⁻ fractions. Genomic DNA from these fractions was subjected to PCR using primers specific for the Arf^Cre and Arf⁺ alleles. Two GFP⁺ samples exhibited virtually complete loss of heterozygosity.