This clinical trial is studying a new way to help people with blood cancers like leukemia. When getting a stem cell transplant from a donor who isn't a perfect match, patients often take a drug called *Cyclophosphamide* to prevent Graft Versus Host Disease (*GvHD*), where the new cells attack the body. This study looks at using a lower dose of Cyclophosphamide after the transplant to see if it still protects against GvHD while causing fewer infections. **Key Points:** - Participants must be between 18 and 66 years old and able to attend all study visits. - The study involves a specific type of stem cell transplant from a partially matched donor. - People who have had previous transplants or have uncontrolled infections cannot join. The study aims to find out if the lower dose can keep patients safe while reducing side effects like infections. Participants will be monitored closely to ensure their health and safety throughout the trial.