Mnt deletion increases apoptosis but does not affect MYC protein level, cell cycling, or senescence in premalignant Eμ-Myc pro-B and pre-B cells. (A) MNT loss does not change MYC level. Pro-B and pre-B cells were sorted from bone marrow of 4-week-old mice, permeabilized and stained with MYC antibody (blue) or isotype-matched control (red). (Bottom) Mean intracellular MYC fluorescence (MFI) ± SD in WT (light green), Eμ-Myc (red), and Mnt^fl/fl Eμ-Myc/Rag1Cre (blue) pro-B and pre-B cells; n = 3 or 4; mean ± SD; **P ≤ .01; ns = not significant. (B) Cell cycle analysis. DNA content of 4′,6-diamidino-2-phenylindole-stained cells shows that the proportion of cycling (S-G2-M) pre-B cells in Mnt^fl/fl Eμ-Myc/Rag1Cre (blue) is comparable to that in Eμ-Myc (red) and Eμ-Myc/Rag1Cre (purple) mice. n = 3-6. Mean ± SD; *P ≤ .05; **P ≤ .01. (C) MNT loss does not significantly affect H3K9 trimethylation. Representative H3K9me3 staining of sorted permeabilized pro-B and pre-B cells from 4-week WT (light green), Eμ-Myc (red), and Mnt^fl/fl Eμ-Myc/Rag1Cre (blue) mice (left panels) and MFI for 3 mice of each genotype in 3 independent experiments (right panels). (D) MNT loss markedly elevates apoptosis. Annexin-V+ pro-B and pre-B cells were 2- to 3-fold more frequent in bone marrow of Eμ-Myc (red) than WT (light green) mice and ∼3-fold higher in Mnt^fl/fl Eμ-Myc/Rag1Cre (blue) than in Eμ-Myc mice. Top panels show typical fluorescence-activated cell sorting plots, and bottom panel shows mean percentage annexin-V⁺ cells; n = 6 to 9 individual mice; mean ± SD; **P ≤ .01; ***P ≤ .001.