Quantitative β-gal histochemical analysis of CMVLacZ and TnILacZ1 expression patterns. The histograms show the optical densities of X-Gal histochemical reaction product measured in individual typed muscle fibers. Each panel corresponds to a different muscle/region. In each muscle or region ~ 70 fibers in a contiguous patch were measured. Three histogram bars are shown for each fiber type in each muscle region, representing CMVLacZ line 9 (black), CMVLacZ line 18 (white), and TnILacZ1 line 36 (gray). Error bars show standard deviations. CMVLacZ expression differences among the fiber types were statistically significant (P < 0.01; t test for soleus, ANOVA for other muscle regions) in every case except for CMVLacZ line 18 plantaris muscle.