**131I-apamistamab-based Conditioning for Hematopoietic Stem Cell Transplant in Advanced Sickle Cell Disease** This study aims to find the smallest amount of a new drug, *131 I-apamistamab*, needed to help people with severe sickle cell disease (SCD) get ready for a bone marrow transplant. A bone marrow transplant can help people with certain diseases by replacing unhealthy blood cells with healthy ones. Before the transplant, patients go through a process called a "conditioning regimen" to prepare their bodies. In this study, researchers will use 131 I-apamistamab instead of the usual treatments like chemotherapy and radiation. The goal is to reduce potential long-term side effects, like cancer or lung problems, from the usual treatments. - **The study will last until 24 patients are enrolled.** - **Participants must be between 12-50 years old and have a matched sibling donor.** - **The drug is investigational, meaning it is not yet approved by the FDA for medical use.** This study could help find a safer way to prepare for stem cell transplants in people with advanced SCD.