The purpose of this research study is to learn more about the safety and effects of CTX001 (the “Study Product”) in patients ages 12 to 35 years who have been diagnosed with severe sickle cell disease. This is an open-label study in which subjects will receive one dose of the Study Product. The goal of is to see if a single “dose” of the Study Product allows the body to increase the amount of hemoglobin F (HbF) while decreasing the effects of SCD. CTX001, which is created by changing (editing) the DNA in the subjects blood stem cells near a gene called BCL11A