 The TEL-AML1 gene fusion results from a karyotypically cryptic t(12;21) translocation, the most common genetic abnormality in pediatric acute lymphoblastic leukemia (ALL).     ///    We demonstrate that the TEL-AML1 translocation can occur in vivo during B-cell development before rearrangement of the IgH and TCR genes.     ///    "The presence of the TEL-AML1 fusion in utero, its protracted latency to overt leukemia, and secondary loss of the untranslocated TEL suggest it is an initiating event. "    ///    TEL-AML1 fusion precedes differentiation to pre-B cells in childhood acute lymphoblastic leukemia..     ///    Sequences of the TEL-AML1 genomic breakpoint and the immunoglobulin heavy chain (IgH) and/or T-cell receptor (TCR) gene rearrangements were characterized in four pediatric pre-B ALL patients.     ///    "Analysis of these markers in relapsed patients revealed that immunophenotypically and cytogenetically distinct, and clonally unrelated antigen receptor leukemic cell populations harbored the same initiating TEL-AML1 molecular abnormality. "