Mer knockdown confers susceptibility to apoptosis induced by serum starvation. Wild-type, shControl, and Mer knockdown (shMer1A, shMer1B) 697 cells were incubated in the presence (Untreated) or absence (24-hour serum-starved) of serum for 24 hours. Apoptotic and dead cells were identified by flow cytometric analysis of cells stained with YO-PRO-1 and propidium iodide. (A) Representative flow cytometric profiles are shown. Cells undergoing apoptosis are stained with YO-PRO-1 but are impermeable to propidium iodide. Dead cells and cells in late apoptosis are permeable to both dyes. Viable cells are not stained by either dye. The percentages of apoptotic (bottom right quadrants) and dead cells (top right quadrants) are shown. (B) Cumulative data demonstrate a significant accumulation of apoptotic cells when Mer knockdown cells are serum-starved for 24 hours (n = 5-6; *P < .05, **P < .001 vs shControl, 2-way repeated measures ANOVA followed by Bonferroni posttests). No significant differences between WT and shControl cells were observed. Mean values and standard errors derived from at least 3 independent experiments are shown.