J Hematol

Journal of Hematology, ISSN 1927-1212 print, 1927-1220 online, Open Access

Article copyright, the authors; Journal compilation copyright, J Hematol and Elmer Press Inc

Journal website https://jh.elmerpub.com

Original Article

Volume 15, Number 2, April 2026, pages 99-107

Abnormal Eosinophils With Large, Distinctly Basophilic Granules (Harlequin Cells) on Peripheral Blood Smear: A Clue for Diagnosing Chronic Myeloid Leukemia

↓ Figure 1. Mimics of Harlequin cells. (a) Eosinophilic myelocyte in reactive bone marrow. (b) Eosinophil with purplish-orange granules. (c) Eosinophil with black granules. (d) Eosinophil with brown granules.

↓ Figure 3. Harlequin cells in chronic myeloid leukemia (CML), acute myeloid leukemia (AML), non-CML myeloproliferative neoplasms and related diseases (non-CML MPN), eosinophilia, and reactive cytosis groups. An overall assessment of the presence or absence of Harlequin cells was determined for each case. Shown are the percentages (%) of cases with Harlequin cells. The positive rate of Harlequin cells in the CML group was significantly higher than that in each of the other groups. However, no significant differences were observed in pairwise comparisons between the other groups.

↓ Figure 4. Harlequin cells in acute myeloid leukemia (AML) with or without (w/o) CBFB::MYH11 fusion. An overall assessment of the presence or absence of Harlequin cells was determined for each case. Shown are the percentages (%) of cases with Harlequin cells. The positive rate of Harlequin cells in AML with CBFB::MYH11 fusion was significantly higher than that in AML without CBFB::MYH11 fusion.

↓ Figure 7. Examples of Harlequin cells in acute myeloid leukemia (AML) without CBFB::MYH11 fusion. (a) Harlequin cells in AML with DEK::DUP214 fusion. (b) Harlequin cells in AML defined by differentiation.

↓ **Table 1.** Summary of Cases
Group	Diagnosis/phase	Number of cases	Incidence of Harlequin cells, n (%) of cases
Chronic myeloid leukemia (CML) (n = 53)	Chronic phase	44	33 (75%)
	Accelerated phase	7	4 (57%)
	Blast phase	2	1 (50%)
	Total	53	38 (72%)
Non-CML myeloproliferative neoplasm and related disorders (n = 30)	Polycythemia vera	5	0 (0%)
	Essential thrombocythemia	5	0 (0%)
	Primary myelofibrosis	15	3 (20%)
	SH2B3-associated erythrocytosis	1	0 (0%)
	Chronic myelomonocytic leukemia	4	0 (0%)
	Total	30	3 (10%)
Acute myeloid leukemia (AML) (n = 59)	AML with CBFB::MYH11 fusion	3	2 (67%)
	AML with GATA2::MECOM fusion	2	1 (50%)
	AML with DEK::DUP214 fusion	1	0 (0%)
	AML with KMT2A rearrangement	2	0 (0%)
	AML with RUNX1::RUNX1T1 fusion	4	0 (0%)
	AML with NPM1 mutation	10	0 (0%)
	AML, myelodysplasia-related	17	0 (0%)
	AML defined by differentiation	15	1 (7%)
	AML post cytotoxic therapy	5	0 (0%)
	Total	59	4 (7%)
Eosinophilia (n = 11)	Myeloid neoplasm with PDGFRA rearrangement	2	0 (0%)
	Lymphocytic variant of hypereosinophilic syndrome	1	0 (0%)
	Idiopathic hypereosinophilic syndrome	6	1 (17%)
	Reactive eosinophilia	2	0 (0%)
	Total	11	1 (9%)
Reactive cytosis (n = 24)	Erythrocytosis/polycythemia	8	0 (0%)
	Thrombocytosis	6	0 (0%)
	Leukemoid reaction	7	0 (0%)
	Leukocytosis + thrombocytosis	3	0 (0%)
	Total	24	0 (0%)

↓ Table 1. Summary of Cases

Group

Diagnosis/phase

Number of cases

Incidence of Harlequin cells, n (%) of cases

Chronic myeloid leukemia (CML) (n = 53)

Chronic phase

33 (75%)

Accelerated phase

4 (57%)

Blast phase

1 (50%)

Total

38 (72%)

Non-CML myeloproliferative neoplasm and related disorders (n = 30)

Polycythemia vera

0 (0%)

Essential thrombocythemia

0 (0%)

Primary myelofibrosis

3 (20%)

SH2B3-associated erythrocytosis

0 (0%)

Chronic myelomonocytic leukemia

0 (0%)

Total

3 (10%)

Acute myeloid leukemia (AML) (n = 59)

AML with CBFB::MYH11 fusion

2 (67%)

AML with GATA2::MECOM fusion

1 (50%)

AML with DEK::DUP214 fusion

0 (0%)

AML with KMT2A rearrangement

0 (0%)

AML with RUNX1::RUNX1T1 fusion

0 (0%)

AML with NPM1 mutation

0 (0%)

AML, myelodysplasia-related

0 (0%)

AML defined by differentiation

1 (7%)

AML post cytotoxic therapy

0 (0%)

Total

4 (7%)

Eosinophilia (n = 11)

Myeloid neoplasm with PDGFRA rearrangement

0 (0%)

Lymphocytic variant of hypereosinophilic syndrome

0 (0%)

Idiopathic hypereosinophilic syndrome

1 (17%)

Reactive eosinophilia

0 (0%)

Total

1 (9%)

Reactive cytosis (n = 24)

Erythrocytosis/polycythemia

0 (0%)

Thrombocytosis

0 (0%)

Leukemoid reaction

0 (0%)

Leukocytosis + thrombocytosis

0 (0%)

Total

0 (0%)

↓ **Table 2.** Demographic Features of Selected Cases
	CML	Non-CML myeloproliferative neoplasm and related disorders	AML	Eosinophilia	Reactive cytosis
CML: chronic myeloid leukemia; AML: acute myeloid leukemia.
Age (years), median (range)	47 (9–75)	65 (20–89)	54 (16–83)	42 (6–89)	55 (20–88)
Sex, n
Male	33	17	38	6	16
Female	20	13	21	5	8

↓ Table 2. Demographic Features of Selected Cases

CML

Non-CML myeloproliferative neoplasm and related disorders

AML

Eosinophilia

Reactive cytosis

CML: chronic myeloid leukemia; AML: acute myeloid leukemia.

Age (years), median (range)

47 (9–75)

65 (20–89)

54 (16–83)

42 (6–89)

55 (20–88)

Sex, n

Male

Female