The primary purposes of this study are to:
•Provide access to cord blood units for recipients whose best choice for a cord blood unit(s) do not meet all FDA standards, but do meet standards set by the NMDP on this study.
•Assess how well and how quickly blood counts return to normal after transplant in recipients on this study.

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.

In this study, the participants will take pembrolizumab for 24 months or get pembrolizumab plus a course of radiation therapy for 24 months until the cancer progresses. After the study treatment is finished, the study doctor will continue to follow the participants condition for a total of 3 years. Some risks in this study is that pembrolizumab with radiation may not be as good as the usual approach for cancer shirking or stabilizing. Some risks associated with pembrolizumab and radiation which are reduction in blood counts, mouth or throat pain and difficulty swallowing. The potential benefit of this study is that radiation and chemotherapy/pembrolizumab can be effective at shrinking or stabilizing the cancer. The purpose of this study is to compare using pembrolizumab with radiation to pembrolizumab without radiation.