Multi-Ctr PII Cmb.Modality Tx Ruxolitinib, Decitabine, and DLI for Post HSCT in AML/MDS

Patient Inclusion Criteria: * Age ≥12 years * Have undergone first allo-HCT from a 6/6 matched sibling donor, 8/8 matched unrelated donor, or an HLA haploidentical donor. * History of AML or MDS for which allo-HCT was performed. Overlap MPN/MDS is included. * Untreated relapse of the underlying malignancy as defined by \>5% of malignant blasts (by morphology and/or flow cytometry) in the bone marrow, or myeloid sarcoma. * Additional cells sufficient for the first DLI available from the same donor, or the donor must be willing to donate. Both G-CSF mobilized and unmobilized products are allowed and the choice is at the discretion of the treating physician. * Partial (or better) engraftment from the bone marrow showing relapse, defined as \>50% donor chimerism on non-split RFLP. Patients with chimerism of 25-50% may be enrolled with approval of the site PI and Sponsor/Investigator * Karnofsky performance status ≥ 50% * Adequate organ function within 14 days of study registration defined as: * Total bilirubin \< 1.5 x upper limit of institutional normal, unless a diagnosis of Gilbert's disease * AST/ALT \< 2.5 x upper limit of institutional normal * Creatinine clearance ≥40 mL/minute as calculated by the Cockcroft-Gault formula. Cockcroft-Gault CrCl = (140-age) \* (Wt in kg) \* (0.85 if female) / (72 \* Cr). * Peripheral white blood cell count \<50 x 10\^9/L. The use of hydroxyurea for cytoreduction is allowed and may continue until cycle 2 day 1 * Subjects and spouse/partner who are of childbearing potential must be using highly effective contraception consisting of 2 forms of birth control (at least 1 of which must be a barrier method) starting at Screening and continuing through the entire study (for at least 3 months after the last dose of ruxolitinib if study treatment is stopped early or subject withdraws consent). Highly effective contraception is defined as: * Established use of oral, injected or implanted hormonal methods of contraception. * Placement of an intrauterine device or intrauterine system. * Double barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository (double barrier method will count as 2 forms of contraception). * Male subjects must not donate sperm during the Screening and Treatment Periods, and for at least 3 months after the last dose of ruxolitinib. * Subjects are willing and able to give written informed consent and to comply with all study visits and procedures. Parents or legal guardians will consent for minors and minors will be asked to assent. Patient Exclusion Criteria: * Active uncontrolled infection at the time of consent. An active uncontrolled infection is defined as hemodynamic instability attributable to sepsis or new symptoms, worsening physical signs or radiographic findings attributable to infection. Persisting fever without signs or symptoms of infection will not be interpreted as an active uncontrolled infection. Subjects with a controlled infection receiving definitive therapy for 72 hours prior to enrollment are eligible. * History of infection with human immunodeficiency virus (HIV), unresolved hepatitis B, or hepatitis C. * Untreated CNS leukemia * Untreated or active GVHD (acute or chronic) * History of grade III-IV acute GVHD at any point in time * Any form of iatrogenic immunosuppression except \<0.5 mg/kg/day of prednisone or equivalent at the time of consent. * Unresolved veno-occlusive disease of the liver, defined as persistent bilirubin abnormalities not attributable to GVHD and ongoing organ dysfunction (renal, ascites). * Subjects who are pregnant, breast feeding or sexually active and unwilling to use effective birth control for the duration of the study, as agents in this study are in pregnancy category C: Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, and category D: there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans. * Radiation therapy within 14 days prior to consent. * Any prior therapy for relapse after allo-HCT. * Prior DLI. CD34-selected boost is allowed * Exposure to any other investigational agent, device or procedure within 14 days prior to consent * Patients or donors with any medical or psychological condition that, in the opinion of the Investigator, might interfere with the subject's participation in the trial, pose any additional risk for the patient/donor, or confounds the assessments of the subject. * Subjects with known allergies, hypersensitivity or intolerance to ruxolitinib or similar compounds.