This study compares two different drug combinations used in stem cell transplants for certain blood cancers. Stem cells are special cells that can become different types of cells in the body. The study involves using either *tacrolimus/methotrexate/ruxolitinib* or *post-transplant cyclophosphamide/tacrolimus/mycophenolate mofetil* to help the body accept new stem cells. **Tacrolimus, methotrexate, and cyclophosphamide** are medications that help suppress the immune system, while **ruxolitinib** and **mycophenolate mofetil** are used to reduce inflammation and prevent organ rejection. **Key Points:** - The study is for adults aged 18 or older with certain blood cancers, like leukemia or lymphoma. - Participants must have a related or unrelated donor match for the stem cell transplant. - The study requires a commitment to using effective birth control methods for 15 months post-transplant. Participants must not have had a previous stem cell transplant and cannot have active infections like tuberculosis or HIV with high viral loads. The study involves careful planning and monitoring of health conditions to ensure safety and effectiveness. Participants will be required to provide consent and can withdraw at any time.