Table 2:

Immunohistochemical and molecular findings in RREB1::MRTFB fusion positive extra-glossal neoplasms

No	Case/reference	Reported as	Histology patterns	Mitoses/10 hpfs	Positive IHC	Negative IHC	Fusion findings
1	Siegfried et al, 2018 [13]	Biphenotypic sarcoma	Monomorphic spindle cells fibrosarcoma-like	4	SMA, S100, desmin, myogenin	h-caldesmon, SOX10, EMA, p63, MUC4, AE1/AE3, CD34, STAT6, MDM2	RREB1 exon 8/ MRTFB exon 11
2	Makise et al, 2020 [14]	Unclassified/extra-glossal EMCMT	Round > > spindle	<1	S100, GFAP, SMA, EMA, AE1/3, ER	Desmin, myogenin, SOX10, HMB45, MUC4, STAT6, CD34, BCOR, NKX2–2	RREB1 exon 8/ MRTFB exon 11
3	Makise et al, 2020 [14]	Unclassified/extra-glossal EMCMT	Spindle >> round > myxoid	<1	S100, GFAP, SMA, EMA, AE1/3, pan-TRK, ER	AE1/AE3, desmin, myogenin, CD34, STAT-6, MUC4, SOX10, GLUT1, claudin-1, MDM2, DOG1, SSTR2a, ALK	RREB1 exon 8/ MRTFB exon 11
4	Bubola et al (2020) [15]	EMCMT	EMCMT	0	SMA, S100, CD56, desmin, GFAP	SOX10, keratin (AE1/ AE3), chromogranin, calponin	RREB1 exon 8/ MRTFB exon 11
5	Mechtersheimer et al, 2021 [16]	Biphenotypic sarcoma	Monomorphic spindle cells fibrosarcoma-like	1	SMA, S100, EMA, CD34	Desmin, myogenin, GFAP, STAT6, AE1/AE3	RREB1 exon 8/ MRTFB exon 11
6	Current Case 1	Unclassified fibromyxoid neoplasm	Fibromyxoid, perineurioma-like whorls	5	CD34, EMA (F+), claudin1 (F+)	Pankeratin, STAT6, SMA, desmin, GLUT1, S100, SOX10, GFAP, ERG, MUC4, INI1 retained	RREB1 exon 8/ MRTFB exon 11
7	Current Case 2	Unclassified fibromyxoid neoplasm	Fibromyxoid, chondromyxoid lobules	0	S100, CD68, EMA (F+)	Pankeratin, Brachyury, Claudin1, GFAP, MUC4, CD34, SOX10. INI1 retained	RREB1 exon 8/ MRTFB exon 11
8	Current Case 3	Chondromyxoid sinonasal hamartoma	Chondromyxoid hamartoma-like	0	S100, CD56 (focal), SATB2 (wk.)	Pankeratin, ERG, ALK, Brachyury, MUC4, Pan-Trk, SSTR2A, Desmin, MDM2	RREB1 exon 8/ MRTFB exon 11
9	Current Case 4	Unclassified spindle and round cell neoplasm	Monomorphic spindle and round cell, highly cellular	2	Synaptophysin, GFAP, CD56, patchy CD99 and PAX8	MUC4, NUT, Pan-TRK, STAT6, SS18 and ALK, EMA, p63, CK19, TLE1, S100, SMA, SOX10, CD34, CK34BetaE12, desmin, myogenin, Myo-D1, WT1, Chromogranin-A, Calcitonin, TTF1	RREB1 exon 8/ MRTFB exon 11
10	Current Case 5	Low-grade mesenchymal neoplasm with rhabdomyoblastic differentiation and RREB::MRTFB fusion, not RMS	Monomorphic ovoid and round cells	1	MyoD1, desmin (focal), myogenin (focal)	GFAP, S100, betacatenin, CD34, CD117, CD3, CD19, CD45	RREB1 exon 8/ MRTFB exon 11

EMCMT=ectomesenchymal chondromyxoid tumor; F=focal; IHC=immunohistochemistry; hpf=high power field; RMS=rhabdomyosarcoma.