This is a research study to find out if the study drug called BEAM-101 is safe and effective in the treatment of patients with severe Sickle cell disease (SCD). The study drug, BEAM-101 is a new investigational (experimental) therapy that is the first in human use of this drug.

The study medication uses patients' own stem cells that are harvested through apheresis (process where blood is removed, stem cells collected and blood is returned), changed by genetic modification, and transplanted back into the individual through intravenous infusion to treat severe SCD.

Participation in this study is expected to last approximately 24 months, starting at time of screening, through the collection of cells, transplantation of study drug, and 15-month follow-up period. Study visits during the screen and mobilization period will vary depending on the cell collection process, in other words, the mobilization and collection period could be 3 separate visits to harvest adequate stem cells. Subjects are then hospitalized for the conditioning period, transplantation of study drug and engraftment period (when blood counts return to normal). The follow up visits after discharge from the hospital will be monthly for the first 6 months, then every other month until 24 months post transplant period.

At the end of the 24 months, all participants will be asked to enroll in the long-term extension study for a duration of 13 years making the total follow-up period of 15 years.

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.