Thus, the main objectives of this study were: i) to accurately quantify the sHER2 levels in cats using both ELISA and the Dot blot assay; ii) to evaluate the usefulness of measuring sHER2 levels in the diagnosis of FMC overexpressing HER2; and iii) to determine the optimal cutoff value for the ELISA and the Dot blot assay, that would differentiate cats with mammary carcinoma overexpressing HER2 from cats with HER2-negative mammary carcinomas or healthy animals. This evidence concerns the gene ERBB2 and breast carcinoma.