The MLL gene, located on chromosomal band 11q23, is fused to a large number of different partner genes in hematological malignancies. This report describes a case of infant acute biphenotypic leukemia with t(1;15;11;10)(p36;q11;q23;q24). Panhandle polymerase chain reaction (PCR) using cDNA demonstrated the formation of an MLL-MLLT10 fusion transcript, although the 10p12 segment, at which the MLLT10 gene is located, was not involved in the breakpoint of the four-way translocation according to G-banding and spectral karyotyping analyses. Long-distance inverse PCR using genomic DNA revealed that intron 7 of MLL was fused with intron 8 of MLLT10, which was connected with a DNA segment of noncoding region on 15q. In fluorescence in situ hybridization analyses, the duplicated 3' part of MLLT10 was inserted into the component of chromosome 15 on der(11)(q23). In real-time quantitative PCR with primers that recognized the DNA sequence of the two sites of fusion point, the minimal residual disease (MRD) levels changed in parallel with other clinical markers. Furthermore, the level of MRD had already increased before hematologic relapse. The identification and characterization of MLL rearrangement at the genomic DNA level may be useful for MRD quantification.