A differential diagnosis of thymoma metastasis versus lymph node involvement by T-lymphoblastic lymphoma was considered for Cases 2 and 3: MIC-2 antibody (CD99) and TdT proved useful in assessing an immature phenotype of lymphocytes present in lymph node metastases, while the anti-CK 19 staining was useful in confirming the epithelial nature of the metastatic lymph node infiltration. This evidence concerns the gene CD99 and thymoma.