MLL/AF9, a fusion gene generated by the t(9;11) translocation [9] that is responsible for a subset of human acute monocytic leukemia, can transform hematopoietic progenitor cells (HPCs) [10], and the resultant MLL/AF9 leukemia cells can be expanded without limit in vitro in the presence of cytokines. This evidence concerns the gene MLLT3 and acute monocytic leukemia.