The stepwise histopathologic evaluation in our patient reflects the diagnostic complexity of maxillary ES, which initially mimicked fibrosarcoma and undifferentiated sarcoma on routine hematoxylin-eosin (H&E) staining. Only after extended immunohistochemical profiling [showing diffuse CD99 and focal FLI-1 positivity with negativity for epithelial, neural, and myogenic markers] was the diagnosis established. The gene discussed is CD99; the disease is fibrosarcoma.