Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) X-linked infantile nystagmus ?
    ### Response:
    X-linked infantile nystagmus is a condition characterized by abnormal eye movements. Nystagmus is a term that refers to involuntary side-to-side movements of the eyes. In people with this condition, nystagmus is present at birth or develops within the first six months of life.
    