There are, on the one hand, what can be called a constant noise when it comes to transgenic and GM technologies: in research and in public discourse, transgenic animals are portrayed as sources of future salvation from human illnesses. However, on the other hand, there are also striking silences when it comes to ethical and welfare concerns. In this paper, we will address how dilemmas with transgenic mice are handled in practice. These animals become dilemmatic at a cultural level, since they can be understood as boundary walkers – or crawlers – constantly balancing on the fine line between nature and culture, organism and innovation, reality and model, science and technology. They constitute forms of techno-scientific hybrids, and, as such, simultaneously challenge and confirm cultural categories and dichotomies. But at the same time, they are quite ordinary laboratory mice with ordinary needs. There are a number of ethical dilemmas involved in the production and handling of transgenic mice, and the paper discusses how these dilemmas sometimes get silenced, and how a re-instatement of the trans-concept may counteract such silences. The paper builds on field notes and interviews with laboratory workers and members of animal ethics committees in Sweden, and engages with feminist science studies, science and technology studies and human-animal studies.