miRNAs are short non-coding RNA that regulate gene expression at the post-transcriptional level by inhibiting the translation of mRNAs through pairing to their 3’ UTR. The let-7 family of miRNAs regulates cell differentiation during embryogenesis and is responsible for the maintenance of the differentiated state in adult cells. Let-7 miRNA levels are often reduced in malignant tumors and its ectopic expression in cancer cells causes cell growth arrest, reduces invasiveness and down-regulates several oncogenes. To gain a further understanding of their biological functions, the breast cancer cell line MDA-MB-231 was transfected with let-7 miRNA mimics and their effect was examined by qPCR, western blot, flow cytometry and functional assays to determine stem cell characteristics and differentiation. We confirmed that let-7 miRNA mimics reduced cell proliferation, and HMGA2, Cyclin D1, Ras and Lin28A protein level in MDA-MB-231 cells. We found that let-7 miRNA down-regulated the levels of active β-catenin (ABC). We showed for the first time that let-7 miRNAs coordinately induce the enzymes in the serine biosynthesis pathway at the transcriptional level in MDA-MB-231 cells. Furthermore, we found that the protein level of the enzymes is differentially regulated: whereas the first two enzymes of the pathway are up-regulated, the last enzyme, phosphoserine phosphatase (PSPH), is down-regulated.ABC down-regulation by let-7 mimics is consistent with the tumor suppressor role of let-7 miRNA family and represents an additional mechanism by which let-7 interferes with Wnt pathway.The regulation of serine biosynthesis pathway is a novel and unexpected function of let-7 that raises many questions and leads to the exciting emerging field: Metaboloepigenetics.