VavP-MYC transgene expression has multiple effects on thymocytes. (A) Proliferation is increased. Sorted thymocyte subpopulations from healthy young mice were permeabilized, stained for DNA content with propidium iodide (PI), and analyzed by flow cytometry. Representative cell-cycle profiles for CD4⁺ CD8^– T cells show an increased proportion of cells in S phase in VavP-MYC transgenic mice, and the cell-cycle distributions of all the thymocyte subpopulations analyzed is shown in Table 4. (B) Cell size is increased. Forward light-scatter analysis of gated thymocyte subpopulations from healthy young mice (2-week-old MYC17 and 6-week-old MYC10, MYC10^hom, and wild-type mice). Representative examples from 3 mice or more examined are shown; wild-type thymocyte size was comparable at 2 and 6 weeks. Gray filled indicates wild-type; black dashed line, MYC10; black solid line, MYC10^hom; and gray solid line, MYC17. (C) Sensitivity to spontaneous apoptosis is increased. Thymocytes from healthy young mice (2- and 6-week-old wild-type, 2-week-old MYC17, and 6-week-old MYC10 and MYC10^hom mice) were cultured in vitro in simple medium. The percentage of gated thymocyte subpopulations remaining viable (not stained with propidium iodide) after 1 to 3 days was determined by flow cytometry (assays were performed on 2 or more independent cultures from each of 2 or more mice of each genotype; mean percentages ± SEM are for replicate mice).