CMVLacZ transgene expression at the level of individual fibers. Panels A-D are near-serial sections of the medial gastrocnemius complex zone of an adult CMVLacZ (line 9) transgenic mouse. Panel A shows β-gal histochemistry and Panels B-D are treated to identify fiber types: Panel B, immunohistochemistry for IIB myosin heavy chain; Panel C immunohistochemistry for IIA myosin heavy chain; Panel D, ATPase histochemistry after preincubation at pH 4.6, with type I (slow) fibers showing strong stain, type IIA fibers showing weak stain, and type IIX and IIB fibers showing intermediate stain. Eight fibers are identified by fiber type where they appear in each panel; A=IIA, B=IIB, X=IIX. Note that nearby fibers of the same type can show very different β-gal expression levels.