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Infant leukemias are rare entities of acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) that are generally diagnosed in infants under the age of one. Patients often present with bulky hepatosplenomegaly, leukemia cutis, central nervous system symptoms, and hyperleukocytosis. Infant leukemia has specific biological, clinical, cytologic, and cytogenetic features. Cytogenetics is an important factor for diagnosis and risk stratification. Infant B lineage ALL is characterized by a higher white blood cell count, higher incidence of KMT2A rearrangements, and negative CD10. Relapse rate is also higher in these cases. KMT2A rearrangements are clearly associated with poorer outcome in infants with ALL. KMT2A rearrangements in acute myelomonocytic leukemia (FAB M4), acute monocytic leukemia (FAB M5), and acute megakaryoblastic leukemia (FAB M7), are also common in infants, but KMT2A rearrangements are not a significant risk factor for infants with AML. As the diagnosis of megakaryoblastic leukemia is challenging, megakaryocytic markers should be investigated in all infant leukemias. It should be noted that transient abnormal myelopoiesis, a condition sharing the same morphology and immunophenotype with acute megakaryoblastic leukemia, frequently occurs in infants with Down syndrome and the somatic mutation of GATA1 is distinct in this situation.
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