Chromosome analysis is necessary for the diagnosis and risk classification of acute myeloid leukemia (AML). 1,2 Fragmented chromosomes whose origin cannot be identified from an existing autosomal or sex chromosome may be observed, termed marker chromosome (MC). MC is considered to originate from marked genomic instability. 3 It is a relatively common abnormality, and its incidence in AML was reported to be 6.6%-16.1%. 4,5 However, MC is currently not specifically considered in the risk classifications by the refined Medical Research Council (rMRC) 1 or European Leukemia Net (ELN). 2 Although AML with MC (MC+) has a poor prognosis even after intensive chemotherapy, 4,5 its influence on the outcome after allogeneic hematopoietic stem cell transplantation (allo-HSCT) is unclear.