In breast cancer, particularly human epidermal growth factor receptor (HER2)-positive and triple-negative breast cancer (TNBC) subtypes, both metformin and phenformin are found to suppress tumor growth and metastasis by downregulating c-MYC and reducing invasiveness through the suppression of hyaluronan synthase 2 (HAS2) [50–52]. This evidence concerns the gene HAS2 and breast cancer.