 While theoretical chimeras seem to further complicate Steel’s mechanistic account of extrapolation, actual chimeras also raise questions that Steel does not address. Consider, for example, humanized mice that have been engineered to carry a “partial or complete human physiological system” (Macchiarini et al. 2005: 1307). These genetically engineered rodents are supposed to make extrapolation more reliable by simulating a variety of human diseases, e.g., asthma, diabetes, cancer, etc. As Monika Piotrowska (2013) points out, however, this raises a new problem. The question is no longer how an inference from model to target can be justified given existing differences between the two, but rather, in what way should these mice be modified in order to justify extrapolation to humans? Piotrowska has proposed three conditions that should be met in the process of modification to ensure that extrapolation is justified. The first two requirements demand that we keep track of parts and their boundaries during transfer, which presupposes a mechanistic view of human disease, but the third requirement—that the constraints that might prevent the trait from being expressed be eliminated—highlights the limits of using a mechanistic approach when making inferences from humanized mice to humans. As Piotrowska explains,