ENPP1 and breast cancer: Because metformin reduces the SP fraction of breast cancer cells46 and generation of the SP fraction was induced by ENPP1, we hypothesized that metformin attenuates the chemoresistance of the SP fraction through miR-27b-mediated suppression of ENPP1. To test this hypothesis, ENPP1 expression was examined in MCF7-luc anti-miR-27b-DR and ZR75-1-luc anti-miR-27b cells that were incubated with 0.1–10 mM metformin for 72 h.