Acute lymphocytic leukemia (ALL) can be classified into subtypes based on cytogenic studies, flow cytometry and molecular genetic studies. These tests look for foreign substances (antigens) in the cells, and abnormalities in the genes and chromosomes of the cells. The cells are then further classified as either B-cells or T-cells.  Subtypes of ALL that occur in children include:

• B-cell ALL. B-cell ALL accounts for approximately 85 percent of ALL cases in children. Subtypes of B-cell ALL include:
  
  
• Early pre-B ALL. This type is the most common subtype of ALL in children, accounting for between 57 and 65 percent of cases.
  
  
• Pre-B-cell ALL. This subtype accounts for 20 to 25 percent of cases in children.
  
  
• Mature B-cell ALL (Burkitt's leukemia). The least common type of ALL in children, this subtype accounts for 2 to 3 percent of cases. The disease is very similar to Burkitt's lymphoma, and as a result, it is treated differently than most leukemias.
  
  
• T-cell ALL. T-cell ALL accounts for approximately 13 to 15 percent of ALL in children. It typically affects boys more often than girls and generally affects children at an older age than B-cell ALL.

The classification of these subtypes is based on an older classification system known as the French-American-British (FAB) Classification of ALL. Based on the appearance of the cancer cells under a microscope (morphology), this system consists of three subtypes including:

• L1. In this type, the lymphoblasts are small cells. It is the most common type of ALL in children.
  
  
• L2. In this type the lymphoblasts are larger. It accounts for 10 percent of ALL cases in children.
  
  
• L3. Also known as Burkitt type leukemia, this subtype is the rarest.