 CONSOLIDATION THERAPY: weeks 6-13 During the safety phase portion of the study, patients with low-risk or intermediate-risk disease are randomized to arms I or III. Patients with high-risk disease are randomized to arms I, II, III, or IV. safety phase closed for accrual as of 092910 During the efficacy phase portion of the study, patients with low-risk disease are randomized to arms I and III. Patients with intermediate-risk or high-risk disease are randomized to arms I, II, III, or IV. The safety phase ends when the first 20 high-risk patients to receive nelarabine have been evaluated. Patients with DS are nonrandomly assigned to arm I DS patients excluded as of 092910. Patients with induction failure are nonrandomly assigned to arm IV. 