a–g, Human fibroblast HCA2-H15c cells containing chromosomally integrated reporter cassettes (a, c, d, g) or the corresponding cGAS knockout cells (b, e, f) were co-transfected with I-SceI endonuclease (to induce DSBs), pcDNA3.1-HA (HA) (control), HA–cGAS and corresponding mutants, and DsRedN-1 plasmid (transfection control) and assessed for efficiency of homologous recombination. h, HCA2-H15c cells were treated with either DMSO (control) or the importin-β inhibitor importazole (10 μM), followed by analysis of the efficiency of homologous recombination. Values in a–h represent the ratio of the quantity of GFP+ cells (corresponding to successful repair events) to the DsRed+ transfection controls. Data represent the mean ± s.e.m. of n = 3 independent experiments. Student’s t tests (unpaired and two-tailed) were used for statistical analysis. NS, not significant. i, Quantitative PCR analysis of BLK expression in the homologous-recombination-repair reporter fibroblast HCA2-H15c cells, transfected with control shRNA or shRNA targeting BLK. Data represent the mean ± s.e.m. of n = 3 independent experiments.