Degradation of TOPO IIβ in control and RA-treated HL-60 cells. Control cells (lanes 1 through 4), 48-hour RA-treated cells (lanes 5 through 8), and 96-hour RA-treated cells (lanes 9 through 12). Lanes 1, 5, and 9, protein level immediately after labeling with³⁵S-methionine; lanes 2, 6, and 10, labeled protein levels at 2 hours after reincubation in complete medium; lanes 3, 7, and 11, labeled protein levels at 4 hours after reincubation in complete medium; and lanes 4, 8, and 12, labeled protein levels at 6 hours after reincubation in complete medium.