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The cell and gene therapy field is experiencing a renaissance of sorts, with about a dozen products approved in the past few years. However, most of these therapies require physicians to collect and purify cells from patients, manipulate them and deliver them back into the patient. Such ex vivo protocols subject patients to long hospital stays and expensive procedures that can be performed only at specialized medical centers. Two companies are developing tools to perform cell and gene engineering in vivo in hopes that these new technologies could make gene therapy widely available as a one-time injection to a much larger number of patients.