OUTLINE: In cohorts of 3, patients receive escalating doses of decitabine (DAC) IV over 4 hours on days -8 and -7. Busulfan is administered orally every 6 hours on consecutive days -6 through -4. Cyclophosphamide is given by vein (IV) over 1 hour on consecutive days -3 and -2. The maximum tolerated dose of DAC is defined as the dose at which 2 patients experience dose limiting toxicity. Donors receive filgrastim subcutaneously (SQ) daily every 12 hours starting 2-4 days prior to the first stem cell collection and before DAC infusion. Leukapheresis is conducted daily. If insufficient number of cells are collected, blood marrow is harvested for supplementation. Stem cells are infused on day 0. For graft vs host disease prophylaxis (GVHD), patients receive tacrolimus IV beginning one day before stem cell infusion, then orally following tolerance to tacrolimus. Patients intolerant to tacrolimus receive cyclosporine IV beginning on day -2, then orally following tolerance and engraftment. All patients receive methylprednisolone given according to clinical grade of GVHD procedures. For CNS prophylaxis, methotrexate is given intrathecally or intraventricularly monthly, beginning on the second month through the eighth month of treatment. Allogeneic patients are followed until the end of 1 year.