To the best of our knowledge, the expression of FLI1 in thoracic SMARCA4-dUT has not been reported, and awareness of the possibility of FL1I expression is essential in the differential diagnosis of high-grade or undifferentiated malignant neoplasms, particularly in the presence of CD34, one of the stemness markers expressed in SMARCA4-dUT, as these markers are also co-expressed in angiosarcoma. The gene discussed is CD34; the disease is angiosarcoma.