i. Patients in 2nd or subsequent relapse, after at least one standard frontline chemotherapy and one salvage regimen, with BM involvement ii. Relapse after allogeneic HSCT, if at least 100 days post-transplant, if there is no evidence of active GVHD and if the patient is no longer taking immunosuppressive agents for at least 30 days prior to enrollment iii. MRD > 0.1% after either reinduction therapy or any course of consolidation for relapsed ALL