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Fig. 6. Musk and MMT protein are concentrated at synaptic sites, despite uniform mRNA expression in Musk-/-; MCK-Musk and Musk-/-; MCK-MMT mice. (A) Musk mRNA is expressed preferentially in the central, synaptic region of muscle in wild-type mice and at uniform, low levels in muscle from Musk mutant embryos. In Musk-/-; MCK-Musk and Musk-/-; MCK-MMT embryos, Musk and MMT mRNA, respectively, are expressed uniformly in muscle. The pattern of Musk or MMT transcription was assessed by in situ hybridization of intercostal muscle from E18.5 embryos. (B) Sections from muscle of 3-week-old mice were stained with antibodies to Musk (EC, antibodies, 83033, to an extracellular epitope; IC, antibodies, 24908, to an intracellular epitope) and with Texas Red {alpha}-Bungarotoxin to label AChRs. Musk and MMT proteins are concentrated at postsynaptic sites despite uniform expression of Musk and MMT RNAs throughout the muscle. IC does not recognize MMT, as MMT lacks the epitope from the C terminus of Musk.