PMC9760045_01
Gross finding and frozen section microscopic presentation; (A) obviously gelatinous cut surface can be seen; (B) abundant myxoid matrix was seen in the background of the tumor (magnification 40x); (C) the spindle cells have mild-moderate cytologic atypia. Mitotic figures can easily be found (red cycle) (magnification 200x).
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It is located in the deep myometrium and partly penetrated the serosa with a tan-yellow and obviously gelatinous cut surface (Figure 1A). Frozen sections showed that the tumour was composed of fascicular spindle cells and had a destructive invasive pattern of the myometrium, which is associated with abundant myxoid stroma (Figure 1B). Mitotic figures can easily be found in some areas in the section (Figure 1C), but necrosis was not seen.
Histological features of the tumor in postoperative paraffin sections; (A) spindle cells showing infiltrative growth within the myometrium with obvious myxoid matrix and thin-walled vessels (magnification 100x); (B) some tumor cells with moderate cellularity were associated of moderate cytological atypia with enlarged nuclei and high N/C ratio and high mitotic count (magnification 200x); (C) focally tumor was characterized by cytoplasmic vacuolation generated a signet-ring like appearance (magnification 200x); (D) abundant myxoid stroma formed multiple microcysts (magnification 100x); (E) focal collagenous plaques presenting (magnification 100x); (F) benign endometrial glands were present on the tumor surface (magnification 100x).
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The permanent sections showed that most areas of the tumor cells with low cellularity have uniform nuclear features with mild-to-moderate nuclear atypia (Figure 2A), scant to moderate eosinophilic cytoplasm, inconspicuous nucleoli, and visible nuclear mitotic activity (5-6 mitoses/10 HPF of 0.55 mm in diameter). Some areas of the tumor cells with moderate cellularity were associated with moderate nuclear atypia with enlarged nuclei and increased nucleoplasm ratio and high mitotic count (16 mitoses/10 HPF of 0.55 mm in diameter) (Figure 2B). There was a significant and abundant myxoid stroma, with some cells appearing as cytoplasmic signet ring cell change (Figure 2C) or forming microcysts containing myxoid material or floating in mucin (Figure 2D). In some areas of the tumor, spindle cells were compacted and associated with collagenous plaques (Figure 2E). Rarely, benign endometrial glands were present on the tumor surface (Figure 2F). 
Immunostaining results of ZC3H7B-BCOR HGESS; (A) CD10(+) (magnification 100x); (B) cyclinD1 (diffues +) (magnification 100x); (C) BCOR(-) (magnification 100x).
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Immunohistochemically, the tumor cells were diffusely positive for CD10 (Figure 3A) and cyclin D1 (Figure 3B), focal positive for PR and negative for ER, Desmin, SMA, h-Caldesmon, CD34, CD117, DOG1, ALK, pan-TRK and HMB45. Moreover, the tumor was negative for BCOR (Figure 3C). 
FISH analysis confirmed gene rearrangements in BCOR showing split signals.
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Technology, China). The FISH assay showed a positive BCOR gene break probe, indicating that a rearrangement of the BCOR gene had occurred (Figure 4), while FISH for YWHAE, PHF1, JAZF1 and PLAG1 rearrangements were all negative. 
RNA sequencing results show that the BCOR gene exon 6 is fused to the ZC3H7B gene exon 11.
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Genomic testing revealed a reciprocal fusion between BCOR exon 6 and ZC3H7B exon 11 (Figure 5). 