DNMT1 and breast cancer: Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor have shown significant activity on inhibiting the phosphorylation retinoblastoma (RB) tumor suppressor, and subsequently inducing G1 cell cycle arrest in tumor cells.133 Apart from inducing cell cycle arrest, CDK4/6 inhibitor could reduce the activity of DNA methyltransferase 1 (DNMT1) in breast cancer cells, which reduces DNA methylation at ERV sequences and increases the levels of dsRNA within tumor cells.