Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Mosaic trisomy 9 ?
    ### Response:
    What causes mosaic trisomy 9? Most cases of mosaic trisomy 9 occur due to a random event during the formation of the reproductive cells (egg and sperm) or after fertilization has taken place. An error in cell division (called nondisjunction) may cause some eggs or sperm to have an abnormal number of chromosomes.
    