Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Fatal familial insomnia ?
    ### Response:
    Fatal familial insomnia (FFI) is an inherited prion disease that affects the brain and other parts of the nervous system. Prion diseases, also known as transmissible spongiform encephalopathies (TSE), are a group of rare neurodegenerative conditions that occur when abnormal proteins clump together and accumulate in the brain, leading to tissue damage.
    