The muscular hypertrophy (mh), or double-muscle phenotype, is a heritable condition in cattle that primarily results from an increase in number of muscle fibers (hyperplasia) rather than the enlargement of individual muscle fibers (hypertrophy), relative to normal cattle (Hanset et al. 1982). The relative increase in fiber number is observed early in pregnancy (Swatland and Kieffer 1974) and results in a calf possessing nearly twice the number of muscle fibers at the time of birth. The occurrence of double muscling has been observed in several cattle breeds worldwide since it was first documented by Culley in 1807. The breed in which this muscular hypertrophy and its effects have been analyzed most extensively is the Belgian Blue breed, which has been systematically selected for double muscling to the point of fixation in many herds. Domestic animals other than cattle also show dramatic increases in muscle mass. Malignant hyperthermia of pigs with muscular hypertrophy (Brenig and Brem 1992) and muscle hypertrophy of cats associated with a dystrophin deficiency (Gaschen et al. 1992) have been analyzed at the molecular level.