The investigators hypothesize that long-term disease-free survival (DFS) in patients with JMML can be achieved with a treatment of busulfan (BU), cyclophosphamide (CY) and melphalan (L-PAM) followed by hematopoietic cell transplantation (HCT).

Prior to transplantation, subjects will receive BUSULFAN via the central venous line, six times a day for four days, CYCLOPHOSPHAMIDE via the central venous line once a day for two days, and MELPHALAN via the central venous line for one day. Busulfan, cyclophosphamide, and melphalan are given to destroy the subject's leukemia. As well, these drugs will destroy the subject's own immune system to help ensure the new bone marrow takes and grows after transplantation.

On the day of transplantation, bone marrow or umbilical cord blood from the donor will arrive to the bone marrow transplant unit and be transfused via venous line. These new cells will replace the subject's bone marrow.

Inclusion Criteria:

• Patients must have a diagnosis of JMML and fulfill these minimal criteria (International diagnostic criteria for JMML):

  • Leukocytosis (> 13,000) with absolute monocytosis (> 1,000)

  • The presence of immature myeloid cells in the peripheral blood

  • Less than 30% marrow blasts

  • Absence of t(9:22) or BCR-ABL transcript

  • Adequate major organ function including:

    • Cardiac: ejection fraction > 45%
    • Hepatic: no clinical evidence of hepatic failure (e.g. coagulopathy, ascites)
    • Karnofsky performance status > 70% or Lansky score > 50%
    • Creatinine must be < 2 x normal for age

• Written informed consent.

Exclusion Criteria:

• Active uncontrolled infection within one week of HCT.

1 year