Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Rett syndrome ?
    ### Response:
    Rett syndrome is a brain disorder that occurs almost exclusively in girls. The most common form of the condition is known as classic Rett syndrome. After birth, girls with classic Rett syndrome have 6 to 18 months of apparently normal development before developing severe problems with language and communication, learning, coordination, and other brain functions.
    