As noted previously, we sometimes house gnotobiotic mice for short durations in class II biosafety cabinets. The class II biosafety cabinet performs two important functions: It protects the contents of the cabinet from outside contamination, while also protecting the user from infectious agents inside the cabinet. This housing is not germ-free, however, and keeping mice clean in this environment requires strict attention to detail. Strict aseptic technique is needed when handling sterile items, entering materials, or manipulating objects/mice in a class II biosafety cabinet. Also, because the only barrier between the inside of the cabinet and the outside is an airflow barrier, it is essential to take care not to interrupt this barrier any more than absolutely necessary. The airflow is disrupted every time an arm reaches into the cabinet or any time an object or body part obstructs the inflow or outflow zones. The airflow can be interrupted even when someone walks by the cabinet. For this reason, mice housed in biosafety cabinets are easily contaminated, particularly with bacterial spores or other airborne contaminants. That said, the advantage of the biosafety cabinets is that mice are easier to manipulate than in the isolators, and it is easier to prevent cross contamination between cages in the same biosafety cabinet than between cages in the same isolator.