SLC29A4 and cancer: Consequently, the levels of expression of the various transporters (OCTs, PMAT, and MATEs) on cancer cells would determine their sensitivity to metformin and the extent to which metformin can exert its inhibitory effect on cancer cell proliferation, invasion, and metastasis while inducing apoptotic cell death [60,61,62,63,64,65,66,67,69,70].