Mixed-lineage leukemia: A type of childhood leukemia in which a piece of chromosome 11 has been translocated (broken off and attached itself to another chromosome). Children with this type of leukemia have a particularly poor prognosis (outlook). The name comes from the fact that that the gene expression profiles in this disease differ from those seen in ALL and AML. They usually have very high white blood counts and a high frequency of central nervous system involvement. They do not respond at all well to the standard therapies for ALL (acute lymphoblastic or lymphocytic leukemia) and often suffer from early relapse after chemotherapy. The mixed lineage leukemia gene is located at 11q23. Translocations affecting this region may be seen in as many as 80% of infants presenting with acute leukemia in the first 6 months of life. The frequency diminishes between 6 months and 1 year of age, but is still significant. The frequency declines in children and then adults with increasing age but may still be found in 3% to 4% of adults with AML and ALL. The translocation may be difficult to detect with conventional chromosome analysis and more sophisticated analytic techniques may be needed to detect it. Mixed lineage leukemia is a distinct entity, not a subtype of ALL, and may require different therapeutic efforts be made.