The agrin gene codes for a family of basal lamina proteins that differ in function and distribution

We isolated two cDNAs that encode isoforms of agrin, the basal
lamina protein that mediates the motor neuron-induced aggregation
of acetylcholine receptors on muscle fibers at the neuromuscular
junction. Both proteins are the result of alternative splicing of
the product of the agrin gene, but, unlike agrin, they are inactive
in standard acetylcholine receptor aggregation assays. They lack
one (agrin-related protein 1) or two (agrin-related protein 2)
regions in agrin that are required for its activity. Expression
studies provide evidence that both proteins are present in the
nervous system and muscle and that, in muscle, myofibers and
Schwann cells synthesize the agrin-related proteins while the axon
terminals of motor neurons are the sole source of agrin.