Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Childhood Leukemia ?
    ### Response:
    Leukemia is cancer of the white blood cells. It is the most common type of childhood cancer. Your blood cells form in your bone marrow. White blood cells help your body fight infection. In leukemia, the bone marrow produces abnormal white blood cells. These cells crowd out the healthy blood cells, making it hard for blood to do its work.
    