This paper uses the figure of the inbred laboratory mouse to reflect upon the management and mobilization of biological difference in the contemporary biosciences. Working through the concept of shifting experimental systems, the paper seeks to connect practices concerned with standardization and control in contemporary research with the emergent and stochastic qualities of biological life. Specifically, it reviews the importance of historical narratives of standardization in experimental systems based around model organisms, before identifying a tension in contemporary accounts of the (...) reproduction and differentiation of inbred mouse strains within them. Firstly, narratives of new strain development, foregrounding personal biography and chance discovery, attest to the contingency and situatedness of apparently universal biotechnological production. Secondly, discoveries of unexpected animal litters challenge efforts to standardize mouse phenotypes and control the reproduction of murine strains over space. The co-existence of these two narratives draws attention to the importance of and interplay between both chance and control, determination and emergence, and the making and moving of experimental life in biomedical research. The reception or denial of such biological excess reflects the distribution of agencies and the emerging spatialities of the global infrastructures of biotechnological development, with implications for future relations between animal lives and human becomings in experimental practices. (shrink)