Acute lymphocytic leukemia is a cancer of the white blood cells. Precursors to white blood cells are called blasts and are made by the bone marrow but in ALL the blasts are abnormal and do not develop into lymphocytes. Instead, the abnormal blasts or leukemic cells multiply rapidly and reduce the level of other types of blood cells such as red blood cells and platelets. There are three main subtypes (L1, L2 and L3) which are differentiated by the appearance of the cancerous cells: L1 is characterized by small, uniform cancer cells with a round nucleus and very little cytoplasm. L1 has a better prognosis than L2. L1 is the main form in children (about 85%) but is less common in adults (about 30%). See also Leukemia, Lymphocytic, Acute, L1