Loss of heterozygosity of the Ink4a/Arf (Cdkn2a) locus in pre-B-ALL. (A) Southern blot analysis of the Ink4a/Arf locus. WT, wild-type fragment of the Ink4a/Arf locus; KO, knockout configuration of the Ink4a/Arf locus. Loss of the upper WT band is indicative of loss of heterozygosity (LOH) of the Ink4a/Arf locus (see supplemental Figure 6). (B) Anti-Arf immunostains in the LNs of mice of the indicated genotypes. The specificity of Anti-Arf antibody was confirmed in LNs from Ink4a/Arf^−/− mice. Scale bar, 10 µm. (C) Kaplan-Meier survival plot illustrating the impact of homozygous Ink4a/Arf deletion on survival of mice with Kras^G12D mutation in the B-cell compartment. P values were calculated by using the log-rank test.