Humane endpoints for mutant animals should be established on the basis of the ability of the mutant to access and consume food and water, the response of the mutant to stimuli, and the general condition of the mutant (for example, it is excessively underweight, it shows progressive weight loss, it doesn’t groom, it has a hunched posture, or it has sensorimotor deficits).

The specific use of a genetically modified animal will influence the type of endpoint that is described in the animal-use protocol and the circumstances in which an endpoint decision will be implemented. For example, an animal that develops a clinical problem while in a study of the prevention of disease development could potentially be euthanized earlier than one involved in a study of disease therapy. A nonexperimental animal (a breeder or an animal intended for but not yet part of a study) that develops a substantial clinical problem should be euthanized. A maximum holding period should be set to avoid the development of predictable problems in strains of mice that have debilitating phenotypes.

Endpoint issues generally apply to the entire life of genetically modified animals. Therefore, endpoints become relevant both in the context of experimental procedures and with regard to the potential pain or distress that is caused by the genetic modification itself. Care must be taken to provide general endpoints in the animal-use protocol for the period in which the initial colony is being developed and the phenotype of the animals is first characterized, as well as for experimental and nonexperimental animals.

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