MR1 is important for MCK expression in slow- and intermediate-twitch skeletal muscle fibers. (A) Sister sections of tibialis anterior (TA) and soleus muscles from mice carrying the 6.5MCK-β-gal or the 6.5ΔMR1-β-gal transgenes, immunostained with myosin heavy chain (MYHC) fiber type-specific monoclonal antibodies (panels 1, 3, 5, 7, 9 and 11) or activity stained for β-galactosidase (β-gal) expression (panels 2, 4, 6, 8, 10 and 12). Antibodies for different isoforms and fluorophore-labeled secondary antibodies mark the fiber types as follows: slow-twitch fibers (type I), blue; intermediate-twitch fibers (type IIa), red; and fast-twitch fibers (types IIb and IId), green and black, respectively (the black appearance of type IId fibers is due to the absence of any type 1, IIa, or IIb antibody binding). Purplish fibers contain both types I and IIa MYHCs (see Figure 5B, soleus), and fibers with weak red or green staining probably contain mixtures of type IId (no color) + type IIa or type IId + type IIb, respectively (see Figure 5B, TA). Sister sections were stained for β-gal expression (false colored gold). Bars are 0.5 mm. (B) Higher magnification sections indicate differences in β-gal expression between fiber types in transgenic lines with and without MR1. Individual fibers, outlined in white or black to show relative differences in X-gal staining between fiber types (type I = K, L and O; type IIa = C, D, G, I and J; type IId = B, F, H and M; and type IIb = A and E), can be cross-referenced to β-gal expression in sister sections.