SLC29A4 and cancer: Metformin will be useful in suppressing cell proliferation and inducing apoptosis in cancer cells and CSCs that express OCTs, which support the intracellular accumulation of metformin, while, cancer cells, and CSCs that overexpress metformin extrusion transporters (MATE1 and PMAT) will confer resistance to metformin treatment [71,72,73,74].