Immunohistochemical expression of melanocytic antigen PNL2, Melan A, S100, and PGP 9.5 in equine melanocytic neoplasms.

The immunoreactivity of PNL2, Melan A, and protein gene product (PGP) 9.5 was compared with that of S100 protein in 50 formalin-fixed, paraffin-embedded equine melanocytic neoplasms. PNL2, PGP 9.5, and S100 protein were detected in all 50 neoplasms; none expressed Melan A. PNL2 was not expressed in 62 nonmelanocytic tumors (equine sarcoids, schwannomas, carcinomas, sarcomas, endocrine tumors, sex-cord stromal tumors, germ cell tumors, and leukocytic tumors) or in normal tissues other than epidermis. In summary, antibody PNL2 is a sensitive marker of equine melanocytic neoplasms and is more specific than S100 protein or PGP 9.5. In contrast, the monoclonal antibody to Melan A did not react with any of the equine melanomas.