In Genome Biology, researchers at Zhejiang University School of Medicine and other centers in China describe ties between Cas9 single-guide RNA (sgRNA) target residence and double-strand DNA break (DSB) repair pathway selection during CRISPR-Cas9-based genome editing experiments. Through a series of mouse embryonic stem cell experiments, the team found that the strength of Cas9-sgRNA interactions with its targeted sequences influences the balance between the use of classical non-homologous end joining pathways and alternative DNA repair pathways. That, in turn, appears to impact off-target editing effects, the authors say. "As CRISPR/Cas9 genome editing generates highly heterogeneous repair products, the effects of Cas9-sgRNA target residence on DSB repair pathway choices at both on-target and off-target sites may significantly contribute to this mutational heterogeneity," they explain, noting that "[t]arget residence of Cas9-sgRNA modulates DSB repair pathway choices likely through varying dissociation of Cas9-sgRNA from cleaved DNA thus widening on-target and off-target mutational spectra in CRISPR/Cas9 genome editing."