Insertion of MLL sequences into chromosome band 5q31 results in an MLL-AF5Q31 fusion and is a rare but recurrent abnormality associated with infant leukemia. 
MLL gene rearrangements leading to production of MLL fusion proteins are commonly detected in infant leukemia patients; the most common MLL fusion associated with infant leukemia is the MLL-AF4 fusion.  A single case of chromosomal rearrangement leading to production of an MLL fusion with AF5Q31, a gene structurally similar to AF4, has been detected recently in the malignant cells of an infant leukemia patient.  We have identified a second case of MLL-AF5Q31 fusion, arising from an insertion of MLL sequences into chromosome 5, also in an infant leukemia patient.  Because MLL and AF5Q31 are transcribed in opposite orientations, a simple balanced chromosomal translocation cannot produce a fusion protein, and complex chromosomal rearrangements such as insertions and inversions are required to produce an MLL-AF5Q31 fusion protein.  This report demonstrates that chromosomal insertion of MLL sequences is a rare but recurrent abnormality associated with infant leukemia.