Flow cytometric profiles showing FRET measurement and its disappearance as an indicator of caspase activation. CFP, YFP, and FRET signals in the cells transfected with either CFP (1) or YFP (2) alone, or co-transfected with CFP and YFP (3), or in the cells transfected with the caspase-sensitive FRET probe CFP-LEVD-YFP alone in the absence (4), or presence (5) of z-VAD, a caspase inhibitor. The cells transfected with the caspase-sensitive FRET probe showed strong positive FRET. However, in the absence of z-VAD, there was a population with bright FRET and one with diminished FRET, denoted as R1 and R2, respectively. The population with diminished FRET disappeared with the addition of z-VAD to the culture.