KMT2A and acute lymphoblastic leukemia: The underlying biological features of ALL in children <4 years are very diverse, ranging from initial standard-risk features [i.e., B-cell precursor (BCP) ALL with t(12;21) translocation] to very high-risk features such as KMT2A gene rearranged ALL in infants, with HSCT being often indicated in first complete remission (CR1).