Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Holoprosencephaly ?
    ### Response:
    Holoprosencephaly is a disorder caused by the failure of the prosencephalon (the embryonic forebrain) to sufficiently divide into the double lobes of the cerebral hemispheres. The result is a single-lobed brain structure and severe skull and facial defects.
    