Leukemia is a cancer of the blood and bone marrow and is classified into four stages: acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML). ALL is the most common type of leukemia in children, but can occur in adults. It is characterized by an overproduction of immature white blood cells, which can crowd out other types of normal cells in the body. Treatment options vary depending on the stage of the disease, but may include chemotherapy, radiation therapy, stem cell transplants, and targeted therapies.