Reciprocal chromosomal translocations are recurrent features of many hematological malignancies. The cloning of the genes located at the breakpoints of chromosomal translocations in leukemia and lymphoma has led to the identification of new genes involved in carcinogenesis. Molecular studies of the breakpoint of several translocations involving chromosomal band 11q23 led to the cloning of a gene that was named MLL. Based on 7969 cases of acute myeloblastic leukemia (AML) and 1252 cases of acute lymphoblastic leukemia (ALL) taken from the literature, band 11q23 and/or the MLL gene was involved in 5.2% of AML and 22% of ALL. Differences in the frequency and the distribution of translocations were noted according to the type of acute leukemia and age of the patients. Seventy-five different rearrangements involving band 1 q23 have so far been identified, 39 MLL partner genes having been cloned. The fusion of MLL and its partner gene leads to a gain of function of the MLL gene. The accumulating data suggests that the fusion protein affects the differentiation of the hematopoietic pluripotent stem cells or the lymphoid or myeloid committed stem cells by deregulating the HOX gene expression patterns.