Each of the applied apoptosis-inducing conditions resulted in a clear-cut pattern of the 25S degradation intermediates or products. This points to the endonucleolytic nature of the reactions, though exonucleolytic destruction, with certain RNA fragments temporarily protected by compact structures or tight interactions with proteins, cannot be excluded. However, mapping 5 0 and 3 0 boundaries of the major H 2 O 2 -induced cleavage product by primer extension and 3 0 RACE, respectively, confirmed that both ends strictly overlap and thus result from the endonucleolytic cut.