Chromosome band 11q23 is the site of translocations in myeloid and lymphoid acute leukemias, pediatric leukemias, and treatment-induced secondary acute myelogenous leukemia. The translocation breakpoints cluster in a restricted region of the HRX gene resulting in chimeric genes that encode an N-terminal portion of Hrx fused to various partner proteins. Myeloid/lymphoid or mixed-lineage leukemia translocated to 1 (MLLT1) is a nuclear protein with transcriptional transactivation properties that is fused to Hrx in t(11;19) leukemias. The minimal MLLT1 sequence required for transcription activation was narrowed to the C-terminal 90 amino acids.