This study is being conducted to assess safety and efficacy of bone marrow transplant in severe and less severely affected children. In this study, we will transplant bone marrow or cord blood from a HLA-matched brother or sister instead of the currently more common method with a HLA-identical sibling donor. This is a multicenter, single arm trial for patients between 2 and 10 years of age who have less severe HbSS or sickle beta zero thalassemia. Patient must have an HLA identical sibling donor who is less than 10 years old and who does not have HbSS, SC or sickle beta zero thalassemia.

This study if for patients that have a blood disease and it's been determined that the best option for treating that blood disease is a cord blood transplant. Cord blood (CB) is blood that is taken from the umbilical cord and placenta of healthy newborn babies after childbirth. The cord blood collected from a newborn baby is called a cord blood unit. The United States Food and Drug Administration (FDA) considers cord blood to be a biological drug. These are considered "investigational" products. This study will evaluate the safety of administration of the investigational cord blood units by carefully documenting all infusion-related problems.