An interstitial 11q23 deletion proven to be a rearrangement interrupting the MLL gene in an infant with acute myeloblastic leukemia.

Chromosome studies of an infant with acute myeloblastic leukemia (AML), classified as M2 in the FAB nomenclature revealed an unusual karyotype with del(11)(q23) and a marker chromosome resembling a small chromosomal fragment present in all metaphase cells examined. Fluorescence in situ hybridization (FISH) showed the splitting of a YAC probe containing a part of MLL between the del(11) and mar chromosomes. Painting showed that the mar chromosome contained DNA sequences from chromosome 11, but that the centromeric region was not marked by a chromosome 11-specific alphoid probe. The chromosomal breakpoint was located within the MLL gene by Southern blot experiments. The deletion of 11q was thus interstitial. This case illustrates the importance of associating cytogenetics, several FISH techniques, and molecular studies to analyze unusual karyotypes in leukemia.