Fig. 4. MCK-MMT induces ectopic, postsynaptic-like specializations. (A) In Musk^-/-; MCK-MMT mice, AChRs are clustered not only at synaptic sites but also at non-synaptic sites (arrows). Wholemounts of diaphragm muscles from 3-week-old mice were stained with antibodies to neurofilament (green) and synaptophysin (green) to label axons and presynaptic nerve terminals, and with Texas-Red -Bungarotoxin (red) to label AChRs. (B) Nonsynaptic AChR clusters co-localize with other `synaptic' proteins, including rapsyn, ErbB4, Na⁺ channels and utrophin. Sections from muscles of 3-week-old mice were stained with antibodies to postsynaptic proteins and with Texas Red -Bungarotoxin to label AChRs. In muscle from wild-type and Musk^-/-; MCK-Musk mice, nerve terminal staining reliably coincides with AChR clusters. In Musk^-/-; MCK-MMT mice, nerve terminal staining coincides with only a subset of AChR clusters.