The "best" positive control: HL-60 cells treated with >0.15 uM captothecin (CPT) provide a reliable model of apoptosis. Most apoptotc changes (mitochondrial, plasma membrane, DNA fragmentation, nuclear fragmentation) occur during the initial 4 h of the treatment. The advantage of this model is that only S phase cells undergo apoptosis (Del Bino et al., Cancer Res., 51: 1165, 1991). Thus, G1 and G2/M cells, within the same sample may serve as a negative control. The critical point is the cells have to rapidly progress through S phase to be sensitive to CPT. It is a collision between the progressing DNA replication fork and the lesion iduced by CPT that provides the signal inducing apoptosis. Any slowdown in S phase progression, therefore, such as due to higher density of cells in the culture (subconfluency; > 800.000 cells per ml) makes them less sensitive to CPT. 2. The issue as to whether the second "p" is silent is a subject of long and ongoing dispute. Interestingly, it become apparent quite recently that the term was already used by the father of medicine Hippocrates, to describe the falling of the bone fragments during healing , i.e. in the context related to its common use (see Esposti: Cell Death & Differ.; 5: 719, 1998). As a Greek word, it should be pronouced with two "p" (see Funder, Nature, vol . 371, 1994 (Sept. 8 issue)"Apoptosis: two p or not two p". English authors, however, often transform its prononciation to English language suppressing the second "p". Zbigniew Darzynkiewicz
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