Acute lymphocytic leukemia is a relatively aggressive 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. The Philadelphia form of myeloid leukemia carries the poorest prognosis of all the different types. It involves an acquired genetic mutation which results in the production tyrosine kinase which causes too many white blood cells to be produced which results in a shortage of other blood cell types. Treatment is aimed at inhibiting the production of tyrosine kinase. See also Leukemia, Myeloid, Philadelphia-Positive