Cas9 (CRISPR associated protein 9) is an RNA-guided DNA endonuclease enzyme derived from Streptococcus that has been widely used for genome editing in a variety of organisms, including humans. Here, we report that exogenous Cas9 protein can elicit an inflammatory immune response through the release of MIP3 alpha, CD40L, and MPO in primary human peripheral blood mononuclear cells and human monocytic cell lines (THP1). Inhibition of the STING-STAT6 pathway blocks Cas9-induced proinflammatory mediator release. These results suggest that targeting the STING-STAT6 axis may prevent host immune responses in human gene therapy with the CRISPR-Cas9 system. (C) 2018 Elsevier Inc. All rights reserved.