The main differential diagnoses include lymphoma, neuroendocrine tumors, epithelial malignancies, mesothelioma or sarcomas on routine hematoxylin-eosin (H&E) staining, their characteristic immunoprofile—coupled with the pathognomonic FOXL2 c.402C>G mutation—readily facilitates distinction from these neoplasms. The gene discussed is FOXL2; the disease is lymphoma.