Peripheral blood (PB) smear and bone marrow (BM) aspirate findings of first case (A, B) and second case (C, D). (A) Eosinophilia (13.0×10⁹/L) was observed in the PB (Wright-Giemsa, ×1,000). (B) Dysplastic eosinophils (sparse and mixed granules) were increased in the BM aspirate (Wright-Giemsa, ×1,000). (C) Eosinophilia (55.316×10⁹/L) was observed in the PB (Wright-Giemsa, ×1,000). (D) The BM section was packed with eosinophils (hematoxylin-eosin, ×400).

Fluorescence in situ hybridization (FISH) for the FIP1L1-PDGFRA rearrangement, and reverse transcription - polymerase chain reaction (RT-PCR) and sequencing analysis. (A) Schematic representation of the 4q12 region and FISH probe. (B) Loss of the orange signal indicates deletion of the 4q12 region (white arrow) in first case. (C) FISH results for second case. (D) RT-PCR was performed using RNA extracted from the patient's (first case) white blood cells (lane 1). EOL-1 cells were used as a positive control (lane 2). Size marker (M). (E) Sequencing analysis of the RT-PCR products revealed the fusion of FIP1L1, exon 12 and truncated PDGFRA, exon 12 (breakpoint is between the 84^th and 85^th nucleotides of exon 12). FIP1L1, Ensemble Gene ID ENSG00000145216; PDGFRA, Ensemble Gene ID, ENSG00000134853.