<qa>  What is the focus of the study LTF-305?All subjects who complete the study, as well as those who withdraw from the study after receiving bb2121 for reasons other than death or meeting the early termination criteria, will be asked to continue to undergo long-term follow-up in a companion study LTF-305 for up to 15 years after their last bb2121 infusion, with a focus on long-term safety and efficacy. 