The necessity of internucleosomal DNA fragmentation for the complete execution of apoptosis is still controversial. While investigating the apoptotic pathway induced by 1-β-D-arabinofuranosylcytosine (ara-C) in the human T-lymphoblastic leukemia CCRF-CEM (CEM) cells, we could easily retrieve high molecular weight (HMW) DNA fragments with a predominant size of 50 kb. However, under the same circumstances, internucleosomal DNA fragmentation characteristic of apoptosis was undetectable despite estimated heightened caspase-3 activity. Paradoxically, generation of low molecular weight DNA fragments was readily demonstrable by flow cytometric and immunohistochemical evidence in the absence of any detectable DNA ladder formation. These findings present a proof that, within certain contexts, small-sized DNA fragmentation occurring in apoptosis may not necessarily be of the ladder yielding internucleosomal integer multiples' pattern. Our data also add to the evidence that the machinery underlying HMW DNA fragmentation is distinct from that responsible for the internucleosomal one.

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