We report on a 69-year-old woman with B-lineage acute lymphoblastic leukemia. Cytogenetic studies at diagnosis with R banding showed an insertion, ins(4;11)(q21;q13q23). Fluorescence in situ hybridization (FISH) with whole chromosome painting probes confirmed the insertion of chromosome 11 material into chromosome 4. FISH using the MLL probe showed the translocation of the 5' end of MLL into chromosome 4. Since the 5'MLL-3'AF4 fusion transcript was detected by a reverse transcriptase polymerase chain reaction, we concluded that the insertion of part of chromosome 11 split the AF4 gene in two, resulting in the presence of the 5'MLL-3'AF4 fusion gene on the der(4) instead of the der(11), as commonly observed. Our findings stress the value of combining banding cytogenetics with FISH and molecular techniques to better assess rearrangements in leukemia.