Given that miR-124 is preferentially expressed in differentiating and mature neurons and external granule cells of cerebellum are thought to be cells-of-origins of medulloblastomas, we investigated if miR-124 played a role in the development of medulloblastomas. Quantitative expression analysis of 29 medulloblastomas demonstrated significant down-regulation of miR-124 in 21 (72%) tumors by at least 2-fold, with 11 of them exhibiting greater than 10-fold reduced level compared to normal cerebella (P < .01). Ectopic expression of miR-124 in medulloblastoma cell lines, ONS-76 and DAOY, inhibited cell proliferation. Using computational and expression analyses, solute carrier family 16, member 1 (SLC16A1) was identified as a candidate target of miR-124. Transfection of miR-124 resulted in down-regulation of SLC16A1 at both transcript and protein levels. Reporter assay with 3' untranslated region of SLC16A1 cloned downstream of the luciferase gene showed reduced luciferase activity in the presence of miR-124, providing strong evidence that miR-124 is a direct regulator of SLC16A1. Expression analysis further revealed that SLC16A1 transcript was elevated in 26 (90%) of 29 tumors examined. Knockdown of SLC16A1 by siRNA induced cell death in medulloblastoma cells. SLC16A1 functions to efflux lactic acid during aerobic glycolysis. We speculated that inhibition of SLC16A1 function resulted in a decrease of intracellular pH to a lethal level. In conclusion, our study demonstrates that miR-124 deregulation is common in medulloblastomas, and restoration of its function inhibits cell proliferation, suggesting that miR-124 may act as a growth suppressor. Our findings also raise the possibility that the miR-124/SLC16A1 pathway may represent a novel therapeutic target for treatment of malignant medulloblastomas.