To assess whether DKK1 exerts direct effects on tumor cell proliferation, we cultured the PyMT, 4T1, and EO771 tumor cells in the presence of recombinant DKK1 (rDKK1) at various concentrations (0, 50, 100, and 200 ng/ml) for 24, 48, and 72 hours and performed an MTT assay. This evidence concerns the gene DKK1 and neoplasm.