PTEN and breast neoplasm: piR-651 is highly expressed in breast tumor tissues and cells, and binds to PIWIL2 to form a complex that promotes the DNMT1-mediated methylation of the phosphatase and tensin homolog (PTEN) promoter, which promotes cell proliferation and invasion and elevates MDM2, CKD4, and CyclinD1 protein levels, as well as inhibits apoptosis; the disruption of piR-651 results in the opposite effect [158].