Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) X-linked creatine deficiency ?
    ### Response:
    X-linked creatine deficiency is a rare condition that primarily affects the brain. Signs and symptoms generally develop before age 2 and may include mild to severe intellectual disability; delayed speech development, behavioral problems (i.e. autistic features, hyperactivity), and seizures.
    