Abstract
Background: S100 protein has been localized in different normal and tumorous tissues. Furthermore, its use in tumor localization and identification has already been established.

Aims and Objectives: The aim of this study is to determine the localization and distribution of S100 immunoreactive cells in different canine mammary tumors (CMTs).

Materials and Methods: The localization of S100 protein was demonstrated in sections of ten different classifications of CMTs and one normal lactating mammary gland by immunohistochemistry. 10 CMT samples were obtained from tumor resections in different veterinary clinics and hospital in Metro Manila, Philippines, and one normal lactating mammary gland was obtained from an anatomy cadaver for comparison.

Results: S100 was observed to be positive in the endothelial cells of blood vessels in all the samples which make it suitable as a marker for CMT. Other cells that have been observed to be moderate-to-high immunoreactivity are spindle-shaped cells, chondrocytes and stromal cells which may serve as tumor markers. A positive immunoreactivity on some neuroendocrine cells and epithelial cells was also observed in neuroendocrine differentiated carcinoma.

Conclusion: Based on the results in 10 CMT, this study suggests that S100 immunohistochemistry can be used as a marker to confirm CMT.

Key words: Canis familiaris; Immunohistochemistry; Mammary Gland Tumors; S-100 Protein