Post Transplant Cyclophosphamide (Cytoxan) for GvHD Prophylaxis

Inclusion Criteria: * Disease Criteria: patients must meet diagnostic criteria of acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), chronic myeloid leukemia (CML), chronic lymphocytic leukemia (CLL), non-Hodgkin lymphoma (NHL), Hodgkin lymphoma (HL), myelodysplastic syndrome (MDS), myelofibrosis, or severe aplastic anemia. Patients will be allowed on study if they are deemed eligible for allo HCT regardless of remission status. * Age Criteria: 19 to 65 years in age. * Organ Function Criteria: All organ function testing should be done within 28 days of study registration. * Cardiac: Left ventricular ejection fraction (LVEF) ≥ 50% by MUGA (Multi Gated Acquisition) scan or echocardiogram. * Pulmonary: FEV1 (Forced expiratory volume in 1 second) and FVC (Forced vital capacity) ≥ 50% predicted, DLCO (diffusing capacity of the lung for carbon monoxide) (corrected for hemoglobin) ≥ 50% of predicted. * Renal: The estimated creatinine clearance (CrCl) must be equal or greater than 60 mL/min/1.73 m2 as calculated by the Cockcroft-Gault Formula: CrCl=(140-age) x weight(kg) x 0.85 (if female)/72 x serum creatinine (mg/dL) * Hepatic: * Serum bilirubin 1.5 upper limit of normal (ULN) * Aspartate transaminase (AST)/alanine transaminase (ALT) 2.5 ULN * Alkaline phosphatase 2.5 ULN * Performance status: Karnofsky ≥ 70%., * Patient must be informed of the investigational nature of this study in accordance with institutional and federal guidelines and have the ability to provide written informed consent prior to initiation of any study-related procedures, and ability, in the opinion of the principal investigator, to comply with all the requirements of the study. * Patient has a suitable and willing HLA-8/8 matched or 6/8 mismatched (at one allele) unrelated donor identified. Exclusion Criteria: * Non-compliant to medications. * No appropriate caregivers identified. * HIV1 (Human Immunodeficiency Virus-1) or HIV2 positive * Uncontrolled medical or psychiatric disorders. * Uncontrolled infections, defined as positive blood cultures within 72 hours of study entry, or evidence of progressive infection by imaging studies such as chest CT scan within 14 days of registration. * Active central nervous system (CNS) leukemia. * Preceding allogeneic HSCT. * Pregnancy or Breastfeeding.