Stem Cell Study for Sickle Cell Disease This study is testing a drug called motixafortide on people with sickle cell disease (SCD). SCD is a condition where red blood cells are misshapen, causing health issues. The study aims to see if motixafortide is safe and if it helps collect more stem cells, which are cells that can develop into different types of cells and may be used for transplants in the future. Participants will be divided into two groups, based on enrollment time. They will receive motixafortide through a shot under their skin. About 8 hours later, stem cells will be collected. Participants will return 7 to 10 days after the injection for follow-up, and study staff will check in with them about a month later. Key Points: - Participants need to have severe SCD and be 18 or older. - They should be willing and able to donate stem cells. - The study involves receiving an injection and follow-up visits. This study is important for potentially improving future treatments for SCD. Participants should discuss with their healthcare provider to understand the potential benefits and risks.