In fact, exposing the human breast cancer cell line MCF-7 to metformin has been reported to increase let-7 levels, but the underlying mechanism of this metformin effect has never been determined.12 To test whether metformin might effect let-7 levels in endometrial cancer cells, ARK2 cells were incubated with metformin, followed by measurement of both let-7 and H19 RNA levels at 12 and 24 h. Here, H19 is linked to breast cancer.