Spark Therapeutics LLC is developing adeno-associated virus vector gene therapies using a manufacturing process that is able to control the number of empty capsids, which it anticipates will improve the safety and efficacy of its products compared to other viral vector gene therapies.

Empty capsids, or vectors without therapeutic genes included, do not add to the therapeutic effect. Moreover, they contribute to toxicity by flagging cells for destruction by capsid-specific CD8 T cells.