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Background/Aims: In CD34-positive acute myeloid leukaemia (AML), the leukaemia-initiating event likely takes place in the CD34+CD38- cell compartment. CD123 has been shown to be a unique marker of leukaemic stem cells within the CD34+CD38- compartment. The aim of this study was to identify the percentage of CD34+CD38-CD123+ cells in AML blasts, AML CD34+CD38- stem cells, and normal and regenerating bone marrow CD34+CD38- stem cells from non-myeloid malignancies. Methods: Thirty-eight adult de novo AML patients with intention to treat were enrolled after the application of inclusion criteria from February 2012 to February 2017. The percentage of the CD34+CD38-CD123+ phenotype in the blast population at diagnosis was determined using a CD45-gating strategy and CD34+ backgating by flow cytometry. We studied the CD34+CD38-CD123+ fraction in AML blasts at diagnosis, and its utility as a unique phenotype for minimal residual disease (MRD) of AML patients. Results: CD123+ cells were present in 97% of AML blasts in patients at diagnosis (median 90%; range 21-99%). CD123+ cells were also present in 97% of the CD34+CD38- compartment (median 0.8164%, range 0.0262-39.7%). Interestingly, CD123 was not present in normal and regenerating CD34+CD38- bone marrow stem cells (range 0.002- 0.067 and 0.004-0.086, respectively). Conclusion: The CD34+CD38-CD123+ phenotype is present in virtually all AML blasts and it may be used as a unique single phenotype for MRD detection in AML patients.