Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to Refsum disease ?
    ### Response:
    More than 90 percent of all cases of Refsum disease result from mutations in the PHYH gene. The remaining cases are caused by mutations in a gene called PEX7.  The signs and symptoms of Refsum disease result from the abnormal buildup of a type of fatty acid called phytanic acid.
    