ATG Plus PTCy vs ATG for CGVHD Prophylaxis

Inclusion Criteria: 1. The participant is aged ≥ 16 and deemed medically fit per investigator for protocol 2. The participant has either acute myeloid leukemia in 1st or 2nd complete remission (CR), or has myelodysplastic syndrome 3. The participant will receive a blood progenitor cell graft ("HPC, Apheresis") 4. The participant has a related or unrelated donor, who is fully MHC matched with the recipient at HLA-A, B, C and DRB1. 5. The participant meets the transplant centre's criteria for transplantation, using either myeloablative or reduced intensity conditioning. 6. The participant has good performance status (Karnofsky ≥60%) 7. The participant is able to understand and sign the informed consent form 8. Ability and willingness to comply with study procedures and schedule, in the Investigator's opinion. 9. The participant is receiving their first transplant Exclusion Criteria: 1. The participant is HIV antibody positive 2. The participant has a hypersensitivity to rabbit proteins or Thymoglobulin® pharmaceutical excipients, glycine or mannitol. 3. The participant has active or chronic infection (i.e. infection requiring oral or IV therapy) 4. The participant (if female and of childbearing potential) is pregnant or breast-feeding at the time of enrollment 5. The participant (if female and of childbearing potential) does not agree to use an adequate contraceptive method from the time of enrollment until a minimum of one year following transplant3 6. The participant (if male and fertile) does not agree to use an adequate contraceptive method from the time of enrollment until a minimum of one year following transplant3 7. The participant has urinary outflow obstruction 8. The participant is in poor condition (determined per institutional guidelines) 9. The participant has acute leukemia in relapse 10. The participant has myelodysplastic syndrome with \> 10% marrow blasts 11. The participant is having their second transplant 12. The participant is taking T-cell antibody prophylaxis (anti-CD52) 13. The participant is receiving a cord blood graft or T-cell depleted grafts 14. The participant has mixed phenotype acute leukemia 15. The participant has prior malignancies, except resected nonmelanoma or treated cervical carcinoma in situ. Cancer treated with curative intent ≥ 5 years previously will be allowed. Cancer treated with curative intent \< 5 years previously must be reviewed and approved by the sponsors. 16. The participant is in complete remission with incomplete recovery (CRi)