This project examines the entangled ethics and epistemology of humanized mice as a case study in how to make bioethics more responsive to core themes of science and technology studies. ‘Humanized animals’ are laboratory model animals engineered to express human traits that cannot otherwise be studied in a human because of experimental or ethical limitations. By engineering mice to express human versions of genes, scientists are able to track how that trait operates within an organism and infer how that trait might operate in humans. Humanized mice are becoming increasingly important for modeling traits that are considered important to humans’ moral and ontological status, such as brain biology or speech capability. Bioethicists have typically reacted to such experiments with the concern that these practices will make the mice too close to the special moral and ontological status of the human, and thus threaten that fixed and pre-determined category of ‘the human.’ However, drawing on STS-inspired historical and sociological accounts of animal modeling, I argue that humanized mice are but one aspect of an experimental apparatus of animal modeling that has drawn humans and lab animals into a contingent and co-constituting relationship in which their moral and ontological statuses are always in flux. In other words, the scientific practices of humanized animals are actually about producing a relationship of close enough. So how ought bioethics incorporate STS methods in this case? If the scientific practice produces an ongoing re-negotiation of ontological and moral boundaries, should bioethics be primarily focussed on policing those boundaries? In contrast with most bioethics literature, I argue that bioethical inquiry should focus on the conditions and consequences of drawing humans and mice into these ongoing relationships, thus making the moral, epistemic, and ontological apparatus of animal modeling the object of ethical inquiry. I thus advocate for a hybrid STS-applied ethics approach that incorporates both STS’s recognition that science and human values constantly remake each other and applied ethics’ impulse to intervene within scientific practice. In the case of humanized animals, learning how to respond to these contingent relationships and track their consequences for both humans and animals is a far more fruitful route than trying to determine and protect a ‘correct’ boundary between humans and animals. I also suggest that such a shift would be useful in bioethics in general.