Chromosomal region 11q23 is frequently rearranged in acute leukemias. The nature of rearrangements is mostly balanced translocations, but also includes unbalanced translocations, inversions, insertions, and tandem duplications. The MLL gene - a transcriptional regulator - resides in this region, and is frequently involved in reciprocal exchanges with various partner genes. One of the most common rearrangements is t(4;11)(q21;q23), in which a chimeric oncogene is formed between MLL and its translocation partner AF4. The t(4;11)(q21;q23) translocation is present in approximately 10% of acute lymphoblastic leukemia (ALL) patients; most frequently in infant leukemia where it is observed over 80%. The t(4;11)(q21;q23) translocation has also been associated with treatment related leukemia (secondary to epipodophyllotoxins). Different breakpoints in the genes result in multiple MLL-AF4 mRNA products with different sizes. It is important to perform molecular diagnostic screening for the presence of t(4;11), along with t(9;22), t(12;21) and t(1;19) in pediatric ALL to determine prognosis and therapeutic approaches.