Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Fatal familial insomnia ?
    ### Response:
    What causes fatal familial insomnia? Fatal familial insomnia (FFI) is caused by a specific change (mutation) in the PRNP gene. PRNP encodes the prion protein. Although the exact function of this protein is unknown, scientists suspect that it plays an important role in the brain.
    