Hipk knockdown causes cell accumulation in S phase and increased apoptosis. Ter119-negative fetal liver cells infected with shRNA-containing retroviruses were cultured in Epo-containing media. Aliquots of cells were removed at 24, 36, and 48 hours and stained with propidium iodide for FACS analysis of cell cycle, or annexin V and 7-AAD for FACS analysis of apoptosis. (A-B) Cell cycle: percentage of cells from each infection in S phase (A) or G₁ (B); n = 3 (mean ± 2 SEM). *P < .05. **P < .01. (C-D) Apoptosis: percentage of cells that are actively undergoing apoptosis but not dead (C; 7-AAD-negative, annexin V-positive) or dead cells (D; 7-AAD, annexin V–double-positive); n = 3 (mean ± 2 SEM). *P < .05.