Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) X-linked infantile spasm syndrome ?
    ### Response:
    X-linked infantile spasm syndrome is a seizure disorder characterized by a type of seizure known as infantile spasms. The spasms usually appear before the age of 1. Several types of spasms have been described, but the most commonly reported involves bending at the waist and neck with extension of the arms and legs (sometimes called a jackknife spasm).
    