Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Zellweger spectrum ?
    ### Response:
    Zellweger spectrum refers to a group of related conditions that have overlapping signs and symptoms and affect many parts of the body. The spectrum includes Zellweger syndrome (ZS), the most severe form; neonatal adrenoleukodystrophy (NALD), an intermediate form; and infantile Refsum disease (IRD), the least severe form.
    