Snail is a protein that, when it is active induces an invasive phenotype in cancer cells. Inactivation of Snail can occur by phosphorylation of its amino acid residue serine 11 (see attached publication). We have generated a phosphosite specific antibody that targets the protein Snail, when it is phosphorylated at serine 11. This antibody can be utilized as a research tool, but also allows to distinguish between normal breast epithelial tissue (in which the phosphorylation occurs) and invasive ductal carcinoma (in which the phosphorylation is decreased or absent). Therefore, this antibody also may be utilized as a diagnostic tool to detect invasive cancer cells and to determine the invasiveness of tumors. This antibody may also allow predicting the invasive potential of other epithelial tumor types.