CCNE1 and malignant colon neoplasm: Furthermore, the protein levels of PCNA protein (a marker for cell proliferation), cyclin D1 and cyclin E (key regulators of the cell cycle), and MMP-9 (a key regulator of metastasis in colon cancer), as well as TNFα (a marker for extrinsic cell apoptosis) in nude mice were measured by IHC.