Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to histidinemia ?
    ### Response:
    Histidinemia is caused by mutations in the HAL gene, which provides instructions for making an enzyme called histidase. Histidase breaks down histidine to a molecule called urocanic acid. Histidase is active (expressed) primarily in the liver and the skin.  HAL gene mutations lead to the production of a histidase enzyme that cannot break down histidine, which results in elevated levels of histidine in the blood and urine.
    