Sebaceous cell differentiation in a canine oral papilloma.

Papillomas caused by viral infection are well-known tumors in animals. Microscopic features typically include neoplastic epithelium with hyperkeratosis and koilocytes. An 8-y-old castrated male Shih Tzu dog was presented with a small exophytic mass on the external upper lip. The mass was diagnosed as a viral papilloma based on microscopic and immunohistochemical examination. Sebaceous cell differentiation was found in the neoplastic epithelium of the tumor, which is a rare finding in humans or animals.