Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Trisomy 17 mosaicism ?
    ### Response:
    Trisomy 17 mosaicism is a chromosomal abnormality in which there are three copies of chromosome 17 in some cells of the body, rather than the usual two copies. Trisomy 17 mosaicism is one of the rarest trisomies in humans. It is often incorrectly called trisomy 17 (also referred to as full trisomy 17), which is when three copies of chromosome 17 are present in all cells of the body.
    