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But there is also the likelihood that both cancers were caused by a powerful drug, busulfan, which is used to clear bone marrow in order to make space for new cells modified by gene therapy.

Busulfan is known to confer a blood cancer risk, Dr. Tisdale noted.

If not, then busulfan is the likely culprit.

An examination found it was caused by busulfan.

Before the cells are replaced, participants are typically treated with a chemotherapy called busulfan to destroy the remaining diseased stem cells and help the reintroduced, genetically altered cells to take over.