Mechanism of chemoresistance mediated by miR-140 in human osteosarcoma and colon cancer cells. In this study, high-throughput microRNA (miRNA) expression analysis revealed that the expression of miR-140 was associated with chemosensitivity in osteosarcoma tumor xenografts. Tumor cells ectopically transfected with miR-140 were more resistant to methotrexate and 5-fluorouracil (5-FU). Overexpression of miR-140 inhibited cell proliferation in both osteosarcoma U-2 OS (wt-p53) and colon cancer HCT 116 (wt-p53) cell lines, but less so in osteosarcoma MG63 (mut-p53) and colon cancer HCT 116 (null-p53) cell lines. miR-140 induced p53 and p21 expression accompanied with G(1) and G(2) phase arrest only in cell lines containing wild type of p53. Histone deacetylase 4 (HDAC4) was confirmed to be one of the important targets of miR-140. The expression of endogenous miR-140 was significantly elevated in CD133(+hi)CD44(+hi) colon cancer stem-like cells that exhibit slow proliferating rate and chemoresistance. Blocking endogenous miR-140 by locked nucleic acid-modified anti-miR partially sensitized resistant colon cancer stem-like cells to 5-FU treatment. Taken together, our findings indicate that miR-140 is involved in the chemoresistance by reduced cell proliferation through G(1) and G(2) phase arrest mediated in part through the suppression of HDAC4. miR-140 may be a candidate target to develop novel therapeutic strategy to overcome drug resistance.