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Background: Mature B cell acute lymphoblastic leukaemia (BAL) is characterised by French-American-British (FAB)-L3 morphology and the presence of surface immunoglobulin (sIgM) light chain restriction. BAL is also considered as the leukaemic phase of Burkitt lymphoma (BL), in which t (8; 14) (q24; q32) or its variants are related to the myelocytomatosis oncogene (MYC) rearrangement (MYCr) is usually present. However, BAL with lysine methyltransferase 2A (KMT2A, previously called Mixed lineage leukaemia, MLL) gene rearrangement (KMT2Ar, previously called MLLr) is rare.Results: Three BAL patients with KMT2Ar were enrolled between January 2017 and November 2019, accounting for 1.37% of the B-ALL population in our hospital. We also reviewed 24 previously reported cases of BAL and KMT2Ar and analysed the features, treatment, and prognosis. Total 13 males and 14 females were enrolled in our research, and the average age at diagnosis was 19.5 ± 4.95 months old. In these 27 patients, renal, central nervous system (CNS) and skin involvement were existent in 6, 4 and 3 patients, respectively; 26 patients (26/27) showed non-ALL-L3 morphology, while one patient is ALL-L3; overexpression of CD19 was detected in most cases, negative or suspicious expression of CD20 was found in 64% of patients. KMT2Ar was reported, but MYCr was not observed. 25 patients (25/27) achieved complete remission after chemotherapy or Stem cell transplantation. The patients were sensitive to chemotherapy, prospective event-free survival (pEFS) of BAL patients with KMT2Ar who received allogeneic haematopoietic stem cell transplantation (allo-HSCT) was higher than that in patients who received chemotherapy alone (83.33% vs 41.91%).Conclusion: BAL patients with KMT2Ar had unique manifestations, including younger age at diagnosis and overexpression of CD19; expression of CD20 was rare, and MYCr was undetectable. The pEFS was higher in patients undergoing allo-HSCT than in patients undergoing chemotherapy alone. [ABSTRACT FROM AUTHOR]