Topo IIβ but not topo IIα is extensively degraded during the Ara-C–induced apoptosis. Western blot analyses of logarithmically growing HL-60 cells (0 hours) and those treated with 4 μmol/L of Ara-C for 5 and 10 hours (5 and 10 hr, respectively) with topo IIα-specific 8D2 (A) and topo IIβ-specific 5A7 and 3G3 monoclonal antibodies (B and C, respectively).