Immune reactions against proteins commonly used as molecular scissors might make CRISPR/Cas9 gene editing ineffective in people, a new study of Stanford University suggested. About 79% of 34 blood donors tested had antibodies against the Cas9 protein from Staphylococcus aureus bacteria, and about 65% of donors had antibodies against the Cas9 protein from Streptococcus pyogenes.

Cas9 is the DNA-cutting enzyme that enables researchers to make precise edits in genes. Antibodies and T cells against the protein could cause the immune system to attack cells carrying it, making gene therapy ineffective.