Agrin signals through the muscle-specific receptor tyrosine kinase (MuSK) to 
cluster acetylcholine receptors (AChRs) on the postsynaptic membrane of the 
neuromuscular junction (NMJ). This stands as the prevailing model of synapse 
induction by a presynaptic factor, yet the agrin-dependent MuSK signaling 
cascade is largely undefined. Abl1 (previously known as Abl) and the 
Abl1-related gene product Abl2 (previously known as Arg) define a family of 
tyrosine kinases that regulate actin structure and presynaptic axon guidance. 
Here we show that the Abl kinases are critical mediators of postsynaptic 
assembly downstream of agrin and MuSK. In mouse muscle, Abl kinases were 
localized to the postsynaptic membrane of the developing NMJ. In cultured 
myotubes, Abl kinase activity was required for agrin-induced AChR clustering and 
enhancement of MuSK tyrosine phosphorylation. Moreover, MuSK and Abl kinases 
effected reciprocal tyrosine phosphorylation and formed a complex after agrin 
engagement. Our findings suggest that Abl kinases provide the developing synapse 
with the kinase activity required for signal amplification and the intrinsic 
cytoskeletal regulatory capacity required for assembly and remodeling.