The goal of this observation study is to test in patients undergoing allogeneic hemopoietic stem-cell transplantation (allo-HSCT). The main question it aims to answer is:

• Effect of MAGIC algorithm probability guided preemption of aGVHD with ruxolitinib on prevention of severe aGVHD.

Participants will take ruxolitinib with the dose of 5mg bid for 28 days. If no signs of aGvHD, the dose of ruxolitinib is gradually tapered within the following 16 days.

Researchers will compare patients who don't receive preemption of aGVHD with ruxolitinib to see if there is an improvement in severe aGVHD.

Inclusion Criteria:

• Any donor type (e.g., related, unrelated, haplo) or stem cell source (bone marrow, peripheral blood, cord blood).
• Any conditioning regimen (non-myeloablative, myeloablative, or reduced intensity) is acceptable.
• GVHD prophylaxis must include a calcineurin inhibitor combined with post transplant cyclophosphamide.
• The use of serotherapy to prevent GVHD (e.g., antithymocyte globulin) prior to day 3 post-HCT is permitted
• Direct bilirubin must be <2 mg/dL unless the elevation is known to be due to Gilbert syndrome within 3 days prior to enrollment.
• ALT/SGPT and AST/SGOT must be <5 x the upper limit of the normal range within 3 days prior to enrollment.
• Signed and dated written informed consent obtained from patient or legal representative.

Exclusion Criteria:

• Patients who develop acute GVHD prior to start of study drug
• Patients at very high risk for relapse post HCT as defined by very high disease risk index
• Patients participating in a clinical trial where prevention of GVHD is the primary endpoint
• Uncontrolled active infection (i.e., progressive symptoms related to infection despite treatment or persistently positive microbiological cultures despite treatment or any other evidence of severe sepsis)
• Patients who are pregnant
• Patients on dialysis within 7 days of enrollment
• Patients requiring ventilator support or oxygen supplementation exceeding 40% FiO2 within 14 days of enrollment.
• Patients receiving investigational agent within 30 days of enrollment. However, the Principal Investigator (PI) may approve prior use of an investigational agent if the agent is not expected to interfere with the safety or the efficacy of ruxolitinib