Soft tissue tumor with novel NR1D1-MAML1 fusion in a pediatric case.

We herein describe soft tissue tumor arising in the lower extremity of a pediatric patient. The tumor displayed a unique and wide range of histological features, sheet-like and cohesive growth pattern consisting of enlarged round to epithelioid atypical cells with a large alveolar and pseudopapillary histological architecture, focally mimicking alveolar soft part sarcoma and MiT family translocation renal cell carcinoma. Tumor cells were focally immunoreactive for cytokeratin, S-100, and EMA. RNA sequencing identified a novel in-frame NR1D1 (exon 5)-MAML1 (exon 2) gene rearrangement resulting in the formation of a putative chimeric protein containing the N-terminal C4-type zing finger domains of NR1D1 and the C-terminal MAML1 protein, which was confirmed by subsequent RT-PCR, Sanger sequencing, and FISH assay. To the best of our knowledge, NR1D1-MAML1 fusion has not yet been described in any neoplasms, suggesting the emergence of a novel tumor entity.