Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Myotonic dystrophy ?
    ### Response:
    What causes myotonic dystrophy? Myotonic dystrophy is caused by mutations called nucleotide repeat expansions in either the DMPK gene (in type 1) or the CNBP gene (in type 2). Nucleotide repeat expansions occur when a piece of DNA is abnormally repeated a number of times, which makes the gene unstable.
    