Skeletal muscle-specific recombination as assessed by β-galactosidase activity. The HSA-MCM strain was bred to a lacZ reporter mouse to visually assess tamoxifen-induced recombination. (A) consistent with the Western blot analysis and PCR results, strong β-galactosidase expression (blue precipitate) was observed only in skeletal muscle (Gstn, gastrocnemius; Pln, plantaris; Sol, soleus; TA, tibialis anterior; EDL, extensor digitorum longus; Diaph, diaphragm; EOM, extraocular muscle) and not in the heart (Hrt) or liver following tamoxifen administration. (B) When we used a second lacZ reporter mouse (that contained a nuclear localization signal), β-galactosidase-positive, “blue” myonuclei were observed in Gstn skeletal muscle after tamoxifen treatment, but not in vehicle-treated Gstn. Inset shows enlarged image of labeled nuclei that reside within the muscle fiber.