Acute Lymphoblastic Leukemia (ALL): A rapidly progressive cancer that starts by the malignant transformation of a marrow lymphocyte. ALL is the most common form of childhood leukemia. The transformed, now malignant, cell multiplies and accumulates in the marrow as leukemic lymphoblasts. The lymphoblasts block normal blood cell-formation in the marrow, resulting in insufficient production of red cells, white cells and platelets. The specific chromosome and genic changes in the affected cells can be used to classify ALL. These findings can be used to determine the expected response of that type of ALL to treatment.