Table.

Current Limitations of CRISPR-Cas9 for Therapeutic Use

Precise genome editing with HDR is inefficient, especially in nonproliferating cells.

Restricted to genomic sites with a PAM.

Potential for unintended consequences at the target site, eg, insertion of undesired sequences, large deletions, and chromosomal rearrangements.

Off-target mutagenesis can occur at unintended sites in the genome but is difficult to detect with current technology.

Durable changes to the genome that are currently prohibitive to reverse.

Too large to easily accommodate in AAV vector—the preferred viral vector for human therapeutics.

Requires testing in human or humanized models to accurately assess efficacy and safety.

Immunologic responses against Cas9-expressing cells, especially if AAV is used.

AAV indicates adeno-associated virus; HDR, homology-directed repair; and PAM, protospacer-adjacent motif.