Histopathological and immunohistochemical analysis confirmed a synchronous primary HAMN (CK20+, Villin+, CK7-) of the appendix and a primary borderline mucinous cystadenoma (CK7+, PAX-8+, CK20-) of the left ovary.<h4>Conclusion</h4>This case underscores the diagnostic difficulty in distinguishing synchronous primary mucinous neoplasms from metastatic disease. This evidence concerns the gene PAX8 and mucinous neoplasm.