Harlequin cells were stringently defined as abnormal eosinophils containing both typical eosinophilic granules and large, distinctly basophilic (not purplish-orange) cytoplasmic granules to exclude reactive mimics.<h4>Results</h4>Harlequin cells were identified in 72% (38 out of 53) of CML cases, a frequency significantly higher than in non-CML MPN (10%, P < 0.01), AML without <i>CBFB::MYH11</i> fusion (3.6%, P < 0.01), eosinophilia (0%), and reactive cytosis (0%) groups. The gene discussed is CBFB; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.