Band 11q23 is known to be involved in translocations and insertions with a variety of partner chromosomes. They lead to MLL rearrangement, resulting in fusion with numerous genes. We report here on a 43-year-old man presenting with asthenia and pancytopenia who was diagnosed with acute myeloblastic leukemia FAB-M5. Conventional cytogenetic techniques showed a del(11)(q21). Using a specific probe for fluorescent in situ hybridization, the MLL gene was found to be disrupted, with the 5' region being inserted into the X-chromosome (around bands q24 approximately q25), as confirmed by whole X-chromosome painting. The accumulating data on acute myeloblastic leukemia demonstrate that the 5'-MLL insertion in an X-chromosome is a rare but recurrent abnormality associated with leukemia, not only in infants, but also in adults.