Valeri and colleagues have demonstrated that in colorectal cancer, miR-21 targets and down-regulates human mutS homolog 2 (hMSH2) and 6 (hMSH6), the core mismatch repair (MMR) recognition protein complex, that in turn determines an increased resistance to 5-fluorouracil treatment [128]. The gene discussed is MSH2; the disease is colorectal cancer.