Schematic illustration of the clonality pattern of the antigen receptor gene rearrangements, demonstrating that one original target was conserved but new sequences were identified in 4 children with relapsed ALL. Germ-line gene segments are presented with more than 98% identity. IgH gene targets are shown in white and new sequences are represented as hatched circles. In case 263, the “new” relapse IgH target could be detected by ASO-PCR using a combination of V_H3 family-specific primer and clone-specific primer based on the sequence at relapse.