Chromosomal abnormalities of the 11q23 band occur frequently in various hematopoietic malignant disorders. Because numerous partner chromosomes have been previously described, it is now important to determine the number of genes involved at 11q23 and to clarify the role of the partner genes. Recent efforts in several laboratories have identified a trithorax-related gene that is involved in most of the 11q23 abnormalities. The aim of this review is to summarize the recent data concerning these 11q23 rearrangements and the understanding of their consequences.