S100 protein is not specific for myoepithelial cells in the canine mammary gland

The distribution of protein S100 in the canine mammary gland was studied, this substance having been described as specific for myoepithelial cells. Two immunohistochemical methods, peroxidase-anti-peroxidase complex (PAP) and alkaline phosphatase-anti-alkaline phosphatase complex (APAAP) were used on frozen sections of normal canine mammary gland. Both the myoepithelial and epithelial cells stained positively with PAP. With APAAP, staining was also seen in the controls, presumably because endogenous alkaline phosphatase was not blocked. As protein S100 was shown to be present both in myoepithelial and epithelial cells of the mammary gland, it was concluded that S100 is not a specific marker for myoepithelial cells.