Figure Legends
FIGURE 1:   Postnatal ultrasound (A ) demonstrates a heterogeneous fusiform solid mass (M) arising along the left back. Calipers measure craniocaudal (1) and anteroposterior (2) dimensions. Highly echogenic region subjacent to the mass denotes air at the pleural surface (black arrows ) and a portion of the spinal column is visible cranially (white arrowheads ).  Sagittal fat-supressed T2-weighted MR image obtained at day 1 of life shows a large encapsulated mass arising from the back (arrows ). Low signal material within the mass (asterisk ) suggests a fibrous or hemorrhagic component, whereas the remainder of the mass is composed of higher signal tissue reflecting a larger cellular component.  Preoperative clinical photograph shows a large erythematous back mass with central skin ulceration (asterisk ), the substance of which appears variably hemorrhagic and necrotic on subsequent photograph of the resected specimen (D ).
FIGURE 2: Histological image of tumor at (A ) 40x original magnification, (B, C ) 200x original magnification showing tumor cells characterized by scanty cytoplasm, round nuclei with neuroendocrine-type chromatin, and 1-2 small nucleoli. A subset of tumor cells had increased clear cytoplasm (C ). Tumor cells diffusely express BCOR based on IHC (D ; 200x original magnification). Additional markers such as BRG1, CD56, CD99 (cytoplasmic, granular), cyclin D1, INI1, pan-NTRK, TLE1, and vimentin showed strong diffuse expression; diffuse moderate expression of BCL2, CD117, and nestin (perinuclear dot pattern); focal expression of CD4 (perinuclear dot pattern) and SATB2, and weak focal expression of EMA and MYOD1 were also present. Pan cytokeratin (AE1/3), chromogranin, CD2, CD25, CD34, D2-40, desmin, GFAP, HMB-45, myeloperoxidase, PHOX2B, PLAP, S100, SALL4, and synaptophysin were negative. WGTA data showing somatic copy number changes in all chromosomes. Red regions indicate copy gain: green regions indicate copy loss (E ). Schema of the YWHAE-NUT2MB fusion. B1:breakpoint 1. B2: breakpoint 2. Numbers in transcripts indicate exons (F).