The PBX1/E2A (also named TCF3/PBX1) DNA-FISH Probe is designed to detect the translocation between the PBX1 gene located on 1q23 and the E2A gene located on 19p13, using fluorescence in situ hybridization (FISH). The translocation between the PBX1 and E2A gene is designated as t(1;19)(q23;p13) and occurs in ~ 6% of pediatric and adult acute lymphoblastic leukemia (ALL) cases; as determined by conventional cytogenetics and reverse transcription-polymerase chain reaction. In both pediatric and adult ALL, the translocation is correlated with a negative prognosis. It may occur as a balanced translocation, t(1;19)(q23;p13), or as an unbalanced translocation, der(19)t(1;19)(q23;p13), where only the derivative chromosome 19 is present. The unbalanced translocation, der(19), is the most common form and accounts for 75% of all PBX1/E2A rearrangements. Both balanced and unbalanced translocations are sometimes observed in the same patient as separate clones.