Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Rett Syndrome ?
    ### Response:
    Rett syndrome is a childhood neurodevelopmental disorder that affects females almost exclusively. The child generally appears to grow and develop normally, before symptoms begin. Loss of muscle tone is usually the first symptom. Other early symptoms may include a slowing of development, problems crawling or walking, and diminished eye contact.
    