Abstract: Among other things, the present disclosure provides cells and non-human animals engineered to express an ADAR1 polypeptide or a characteristic portion thereof. In some embodiments, the present disclosure provides cells and non-human animals engineered to express a human ADAR1 polypeptide or a characteristic portion thereof. In some embodiments, non-human animals are genetically modified rodents such as mice, rat, etc. In some embodiments, non-human animals are mice. In some embodiments, the present disclosure provides technologies for assessing an agent comprising administering the agent to a cell or non-human animal engineered to express an ADAR1 polypeptide or a characteristic portion thereof. In some embodiments, such a cell or non-human animal is engineered to express a human ADAR1 polypeptide or a characteristic portion thereof. In some embodiments, an agent is a pharmaceutical agent. In some embodiments, an agent is or comprises an oligonucleotide.