The MLL Break Apart probe is designed to detect the translocation involving the MLL gene on chromosome 11q23 using fluorescence in situ hybridization (FISH). At least 104 translocation partner genes have been identified. Translocation of MLL is found in ~3-10% of acute lymphoblastic leukemia (ALL) cases, and in ~8-10% of acute myeloid leukemia (AML) cases, and is prognostically relevant in these leukemias. However, the prognostic implication is dependent on the age and phenotype of the leukemia. MLL rearrangement has been observed in ~80% of infant ALL cases and is associated with a high risk in such cases and requires aggressive treatment. In AML, the prognosis is intermediate regardless of age. MLL translocations are also found in ~25% of patients with therapy-related leukemias, particularly following treatment with DNA topoisomerase II inhibitors and the prognosis in such patients is poor. In addition to translocations, deletions of 3’ MLL and amplification of MLL also occurs in a subset of ALL and AML cases.