Recurring chromosome translocations, which are found in leukemia, can result in the inappropriate expression of oncogenes or in the formation of chimeric genes that code for structurally and functionally abnormal proteins. The chromosomal t(1;9)(q23.3~q25;q34) was found in a patient with biphenotypic leukemia. Fluorescence in situ hybridization (FISH) analysis revealed that the break on chromosome 9 occurred in the ABL1 gene. The breakpoint on chromosome 1 occurred distal to the PBX1 gene at 1q23.3, as shown by FISH using BAC RP11-503N16 and RP11-403P14, which flank the PBX1 locus; hence, the ABL1 gene can be fused with another gene distal to PBX1 gene.Laboratorio de Citogenética, Centro de Investigación Biomédica de Occidente, Centro Médico Nacional de Occidente, Instituto Mexicano del Seguro Social (CIBO-IMSS), Sierra Mojada #800, Colonia Independencia, Guadalajara, Jalisco AP 1-3838, México Medizinische Klinik und Poliklinik III, Klinikum Grosshadern, Raum 003, Clinical Cooperative Group ‘Leukemia’, GSF, Marchioninistrasse 25, 81377 München, Germany Doctorado en Genética Humana, Universidad de Guadalajara, Guadalajara, México Departamento de Hematología, Hospital de Pediatría, Centro Médico Nacional de Occidente, Instituto Mexicano del Seguro Social (CMNO-IMSS), Sierra Mojada #800, Colonia Independencia, Guadalajara, Jalisco AP 1-3838, México