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Abstract Internal tandem duplication (ITD) of FMS-like tyrosine kinase 3 (FLT3) confers poor prognosis and is found in approximately 25% of cases of acute myeloid leukemia (AML). Although FLT3 inhibitors have shown clinical benefit in patients with AML harboring FLT3-ITD, the therapeutic effect is limited. Here, to explore alternative therapeutics, we established a cellular model of monoallelic FLT3 ITD/WT cells using the CRISPR-Cas9 system in a human myeloid leukemia cell line, K562. cDNA microarray analysis revealed elevated CD52 expression in K562–FLT3ITD/WT cells compared to K562–FLT3WT/WT cells, an observation that was further confirmed by quantitative real-time-PCR and flow cytometric analyses. The elevated expression of CD52 in K562–FLT3ITD/WT cells was decreased in wild-type FLT3 (FLT3-WT) knock-in K562–FLT3ITD/WT cells. In K562–FLT3ITD/WT cells, a STAT5 inhibitor, pimozide, downregulated CD52 protein expression while an AKT inhibitor, afuresertib, did not affect CD52 expression. Notably, an anti-CD52 antibody, alemtuzumab, induced significant antibody-dependent cell-mediated cytotoxicity (ADCC) in K562-FLT3ITD/WT cells compared to K562–FLT3WT/WT cells. Furthermore, alemtuzumab significantly suppressed the xenograft tumor growth of K562–FLT3ITD/WT cells in severe combined immunodeficiency (SCID) mice. Taken together, our data suggested that genetically modified FLT3-ITD knock-in human myeloid leukemia K562 cells upregulated CD52 expression via activation of STAT5, and alemtuzumab showed an antitumor effect via induction of ADCC in K562–FLT3ITD/WT cells. Our findings may allow establishment of a new therapeutic option, alemtuzumab, to treat leukemia with the FLT3-ITD mutation.