Tissue mass is maintained by coordinated regulation of cellular processes, including proliferation, differentiation, and apoptosis. Changes in these processes occur during normal growth and development throughout an animal's lifetime beginning in the embryo. Irregularities in these processes can result in abnormalities, such as double muscling or bone dysplasia. A cell's ultimate phenotype is expressed as cellular structure, shape, and function and is determined during differentiation. Transgenic animals, in which expression of specific genes has been altered, have been especially helpful in determining gene functions in vivo, whereas in vitro studies have been useful in determining these roles in more controlled environments.1-1-1

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