ICN1 inhibits B-cell but not T-cell growth. (A) V9 pre–B-cell line, 70Z pre–B-cell line, JurkatE human T-cell line, and G4A2 T-cell lines were transduced with retrovirus expressing either GFP only (MigR1) or ICN1 and GFP (MigICN1) and analyzed by flow cytometry. Transduced cells were identified as GFP⁺, and the percentage of GFP⁺ cells was plotted against time. (B) ICN1 induces cell cycle arrest. 70Z cells were transduced with the indicated retrovirus, and GFP⁺ cells were sorted 24 hours later. Sorted cells were then plated out (5 × 10⁴ cells/mL). Cells were harvested at 4 time points: 30, 48, 60, and 72 hours after transduction and fixed for PI staining and FACS analysis to determine the cell-cycle profile. The ratio of G₀(G₁)/S is shown. Results are from 3 independent experiments; mean and standard deviation are shown. P < .05 (ANOVA). (C) ICN1 induces apoptosis in pre–B-cell lines but not in T-cell lines. The transduced cell lines described in Figure 1A were assayed for annexin V expression by FACS. The GFP intensity of the annexin V⁺ cells decreased because of apoptosis-induced GFP leakage. (D) ICN1-induced apoptosis is caspase dependent. V9 cells were transduced with MigR1 or MigICN1 as described in Figure 1A. After transduction, ICN1-transduced cells were divided into aliquots into 2 wells in parallel and incubated in the presence of either 50 μM caspase inhibitor z-val-Ala-DL-Asp-fluoromethylketone (zVAD) or dimethyl sulfoxide (DMSO). Cells were collected at 22 hours and 44 hours after transduction and stained for annexin V prior to analysis.