Genetically engineered animals have opened new frontiers in the study of physiology and disease processes. Mutant animals offer more accurate disease models and increased precision for pathogenesis and treatment studies. Their use offers hope for improved therapy to patients with conditions that currently have poor or ineffective treatments. These advantages have fostered an increase in studies using mice in recent years, a development viewed with alarm by those who oppose the use of animals in research. Scientists point out that the mice are replacing more sentient species, such as nonhuman primates, and are increasing the quality of research being conducted. They assert that study of genetically engineered animals will eventually permit decreases in numbers of animals used in research. Nevertheless, the increase in use of genetically altered animals presents many challenges in reviewing protocols and providing care. Identification and resolution of any welfare problems is a responsibility that is shared by institutional animal care and use committee, veterinary, animal care, and research staffs. To identify potential welfare concerns, a database such as TBASE () can be searched to learn what has been reported for established mutant lines. In addition, newly created lines should be monitored by a surveillance system and have phenotype assessment to identify the effects of altering the genome. Methods of ensuring welfare can include treatment of conditions produced, restriction of gene expression to tissues of interest or to certain time periods, and establishment of endpoints for removing animals from a study before problems appear.