A versatile cGAMP reporter reveals principles of cGAS activation by DNA damage and chromosome instability.

cGAS is the primary innate immune DNA sensor. On binding DNA, cGAS generates cGAMP, ultimately driving inflammation. Although normally silenced on self-DNA, genotoxic stress can activate cGAS, proposed to be mediated by micronuclei, chromosome bridges and DNA:RNA hybrids. However, mechanistically, this is poorly understood due to a lack of sensitive and selective single-cell cGAS activation assays. Here we solve this with an improved cGAMP reporter for microscopy, flow cytometry and biochemical assays. Strikingly, we find that genotoxic stress-mediated cGAS activation is a rare event that is not driven by enrichment on micronuclei and occurs by mechanisms that vary in dependence on the genotoxic stress. Following chromosome mis-segregation, cGAS activation correlates with bridge association but, notably, ionizing radiation activates cGAS independently of bridges. Whereas simple DNA:RNA hybrids are inert, more complex structures such as R-loops activate cGAS. Our work revises the cGAS signalling framework and introduces a flexible tool to examine it.