A Study to Assess 7/8 HLA-matched Hematopoietic Stem Cell Transplantation Participants Treated With or Without Abatacept in Combination With a Calcineurin Inhibitor and Methotrexate

Inclusion Criteria: * Participants who underwent first allogenic transplant in the US * Participants with an unrelated donor who are HLA-matched at 7/8 loci (A, B, C, DRB1) * Participants at least 6 years old with weight at least 20 kilograms * Participants with a Karnofsky/Lansky Performance Score ≥80% * Participants whose first allogeneic transplant occurred from January 1, 2011 to December 31, 2018 * Participants with any of the following diseases: AML, ALL, CML, MDS, HL, NHL * Participants with any of the following graft versus host disease (GVHD) prophylaxis treatments: * CNI plus MTX (with or without ATG and with or without abatacept); or * Post-transplant cyclophosphamide (PT-Cy) without antithymocyte globulin (ATG) * Participants treated with any of the following conditioning regiments: total body irradiation (TBI)/cyclophosphamide (Cy), busulfan (Bu)/Cy, Bu/fludarabine (flu), Flu/Melphalan (MEL) Exclusion Criteria: * Participants with missing information on ATG (yes/no) * Participants receiving alemtuzumab (Campath) * Participants with cord blood grafts * Participants with non-MDS myeloproliferative disorders (NOTE: Participants with chronic myelomonocytic leukemia \[CMMoL\] will be included) * Participants who did not consent to participate in research * Participants treated at embargoed centers for research * Participants treated with abatacept and ATG * Among non-abatacept treated participants, participants transplanted at centers with abatacept trial participants * Participants with any of the following missing propensity score variables: * Disease status at transplantation (early, intermediate, advanced HL and NHL-chemosensitive) * Age * Gender (male, female) * HSCT graft source (bone marrow \[BM\], peripheral blood \[PB\]) * Conditioning intensity (myeloablative, non-myeloablative / reduced intensity) * Karnofsky/Lansky Performance Score (80%, 90-100%) * CNI type (tacrolimus, CsA)