Increased caspase-8 activity in early-stage MDS erythroid precursors. Caspase-8 activity on day 14 was measured by the cleavage of the fluorescent substrate IETD-AFC. Median activities (horizontal bars) were 7340 arbitrary units (AU) of fluorescence emission (range, 5930-10 721 AU) in ES-MDS erythroid precursors, 4107 AU (range, 2680-6966 AU) in normal erythroid precursors, and 3832 AU (range, 2805-5780 AU) in AS-MDS erythroid precursors. Boxes represent IQR (0.25-0.75), and error bars indicate upper and lower values. (^**P < .001; ^*P < .01).