Inclusion Criteria:

          -  Patients in 1st or 2nd remission with acute myeloid leukemia (AML) or acute
             lymphoblastic leukemia (ALL), who are eligible for stem cell transplant. Remission
             defined as no circulating blasts, < 5% blasts in the bone marrow, normalization of
             previously detected cytogenetic abnormalities, no extramedullary disease

          -  High risk myelodysplastic syndrome (MDS)

               -  Intermediate II and high risk by International Prognostic Scoring System (IPSS)

               -  Intermediate, high, or very high by World Health Organization (WHO)
                  classification-based Prognostic Scoring System (WPSS)

               -  Transfusion dependent

               -  Therapy-related MDS or MDS evolved from previous hematological disorder
                  (excepting myelofibrosis)

          -  Patients with chronic myelomonocytic leukemia (CMML) are allowed to be enrolled

          -  Patients with MDS that has evolved to AML must be in remission

          -  Patients must not be eligible for full ablative regimens by the attending physician

          -  Patients with AML or MDS arising from myeloproliferative neoplasm can be enrolled
             after principal investigator (PI) approval on case to case basis, depends on the
             spleen size and degree of bone marrow fibrosis

          -  Performance status of >= 70% on the Karnofsky scale

          -  Women of child-bearing potential and men must agree to use adequate contraception
             (hormonal or barrier method of birth control or abstinence) prior to study entry and
             for six months following duration of study participation; should a woman become
             pregnant or suspect she is pregnant while participating on the trial, she should
             inform her treating physician immediately

          -  Bone marrow and peripheral blood studies must be available for confirmation of
             diagnosis; cytogenetics, flow cytometry, and molecular studies (such as Flt-3 status)
             will be obtained as per standard practice

          -  Bone marrow aspirates/biopsies should be performed within 28 (+ 4 day window) days
             from registration to confirm disease remission status

          -  A pretreatment measured creatinine clearance (absolute value) of >= 60 mL/minute

          -  Patients must have a serum bilirubin =< 2.0 mg/dl

          -  Patients must have serum glutamic oxaloacetic transaminase (SGOT) and serum glutamate
             pyruvate transaminase (SGPT) =< 2.5 times the institutional upper limit of normal

          -  Ejection fraction measured by echocardiogram or multi gated acquisition scan (MUGA) >
             50%

          -  Diffusing capacity of the lung for carbon monoxide (DLCO) or forced expiratory volume
             in 1 second (FEV1) > 45% predicted

          -  Availability of a human leukocyte antigen (HLA) matched (6/6) sibling donor or 8/8
             matched unrelated donor; Donors with mismatch at HLA-A, HLA-B, HLA-C, and HLA-DR will
             be reviewed by matched unrelated donor (MUD) committee and allowed if their mismatch
             with the recipient does not require additional GVHD prophylaxis (other than tacrolimus
             and sirolimus), donors with mismatch at HLA-DQ or HLA-DPB are eligible; donor
             evaluation according to City of Hope (COH) standard operating procedure (SOP)

          -  Donor stem cell source can be either peripheral blood or bone marrow

          -  All patients must have a psychosocial evaluation prior to transplant as per COH SOP

          -  All subjects must have the ability to understand and the willingness to sign a written
             informed consent

          -  ALL or AML patients who received chemotherapy (induction or consolidation) can proceed
             to transplant once bone marrow cellularity is > 10 % with no evidence of leukemia

        Exclusion Criteria:

          -  Patients who have received a prior autologous or allogeneic transplant are excluded

          -  Patients with significant hepatic dysfunction (not meeting liver function tests [LFT]
             eligibility criteria)

          -  Patients with MDS evolved into AML that is not in remission

          -  Patients with acute promyelocytic leukemia

          -  Patients with myeloproliferative neoplasms

          -  Patients with suspected or proven central nervous system (CNS) leukemia; (diagnostic
             lumbar puncture not required before enrollment)

          -  Uncontrolled intercurrent illness including, but not limited to ongoing or active or
             poorly controlled infection, symptomatic congestive heart failure, unstable angina
             pectoris, cardiac arrhythmia, poorly controlled pulmonary disease or psychiatric
             illness/social situations that would limit compliance with study requirements

          -  Pregnant and lactating women are excluded from this study

          -  Patients who do not agree to practice effective forms of contraception

          -  Human immunodeficiency virus (HIV)-positive patients are excluded from this study

          -  Patients are excluded if they are hepatitis B surface antigen (sAg), hepatitis B (Hep
             B) core antibody (cAb), or hepatitis C (Hep C) positive. Patients with Hepatitis B cAB
             positive and Hepatitis B PCR negative are eligible if they started prophylactic
             treatment prior to registration to trial

          -  Patients who have received radiation therapy as part of their leukemia treatment may
             be ineligible and individual cases must be presented to the study principal
             investigator (PI) for determination of eligibility

          -  Any psychiatric, social or compliance issues that, in the treating physician's
             opinion, will interfere with completion of the transplant treatment and follow up

          -  Medical or psychiatric reasons which make the donor unlikely to tolerate or cooperate
             with filgrastim (G-CSF) therapy or leukapheresis or bone marrow harvest

          -  Known allergies to clofarabine, melphalan, sirolimus or tacrolimus

          -  Patients with other active malignancies (besides AML, ALL, MDS) requiring treatment or
             where there is concern of progression are ineligible for this study; however, patients
             with previously treated skin cancer, early stage cervical or prostate cancer may be
             eligible if there is no evidence of residual disease

          -  Cord blood as a donor source is not acceptable

          -  Subjects, who in the opinion of the investigator, may not be able to comply with the
             safety monitoring requirements of the study