Cucurbitacin B can reduce the levels of DNA methyltransferase (DNMT1, DNMT3a, and DNMT3b) through the demethylation of the BTG3 promoter in rectal cancer cells and re-activate BTG3.The cell cycle of cancer cells can be blocked in the G1 phase in vitro, and the levels of Cyclin D1 and Cyclin E1 in cancer cells can be significantly reduced. The gene discussed is DNMT1; the disease is cancer.