Inclusion Criteria:  Acute Myeloid Leukemia  Chemo-refractory relapse (MRD+) Primary induction failure (no CR or CRi after >= 2 courses of intensive induction therapy): < 30% blasts in evaluable marrow. Relapse after previous allogeneic (or autologous) transplant (>4 months) Secondary or therapy-related MDS/AML Poor response to induction (5-30% blasts) or MDR+ after induction  Myelodysplastic syndrome (MDS)  Relapse after allogeneic or autologous transplant (>4 months) ≥10 % blasts within 30 days of start of conditioning regimen Poor and very poor cytogenetics abnormalities Chronic myelogenous leukemia: Patients who progressed to blast crisis Mixed Phenotype Acute Leukemia: MRD+ or relapse after previous transplant (>4 months). JMML (Juvenile Myelo-Monocytic Leukemia)  Availability of 2 ≥ 4/8 HLA matched CBU (allele level: A, B, C and DRB1)  Cord to be expanded: CD34+ cell count ≥ 0.5 x 10^5/kg and TNC ≥ 1.5 x 10^7/kg (pre-cryo) Back up cord: Pre-freeze TNC ≥ 2 x 10^7/kg with CD34+ cells ≥ 1.5 x 10^5/kg. If a single cord does not meet this criterion 2 back up cords will be an acceptable alternative with a minimum for each of 1.5 x 10^7 TNC/kg with 1.0 x 10^5 CD34+/kg. Another acceptable HSC back up source could be a haploidentical with medical clearance prior to starting conditioning regimen. Lansky / Karnofsky >60% Bilirubin < 2 x upper limit of normal (ULN) unless felt to be related to Gilbert's disease or hemolysis; AST and ALT < 3 x ULN; alkaline phosphatase < 5 x ULN Estimated or measured creatinine clearance ≥ 50ml/min/1.73m2 Left ventricular ejection fraction of ≥ 40% FVC, FEV1 and DLCO ≥ 50% of predicted Signed written informed consent Female patients of childbearing potential must have a negative serum pregnancy test within 7 days of enrolment and mush be willing to use an effective contraceptive method while enrolled in the study.  Exclusion Criteria:  Previous allogeneic transplantation within 4 months. Uncontrolled infection. Presence of other malignancy other than the one for which the CB transplant is being performed, with an expected survival to be less than 75% at 5 years Seropositive for HIV. Hep B and C infection with measurable viral load. Liver cirrhosis. Active CNS disease. Chloroma > 2cm. >30% blasts in marrow in evaluable marrow sample. Pregnancy, breastfeeding, or unwillingness to use appropriate contraception Participation in a trial with an investigational agent within 30days prior to entry in the study. Any abnormal condition or lab result that is considered by the PI capable or altering patient's condition or study outcome.