Disclosed are DNA encoding a polypeptide which can modulate the activity of a muscle-specific tyrosine kinase, and others. The DNA is selected from the following members (a) to (d): (a) DNA comprising a specific nucleotide sequence; (b) DNA comprising a nucleotide sequence capable of hybridizing with a specific nucleotide sequence under stringent conditions; (c) DNA comprising a nucleotide sequence encoding an amino acid sequence having the substitution, deletion and/or addition of one or several amino acid residues in a specific amino acid sequence; and (d) DNA comprising a nucleotide sequence having 90% or higher homology to a specific nucleotide sequence.