MicroRNAs and cancer drug resistance: over two thousand characters in search of a role.

MicroRNAs (miRNAs), a group of small regulatory noncoding RNAs, transformed our thinking on gene regulation. More than two thousand human miRNAs have been identified thus far. These bind imperfectly to the 3'-untranslated region of target mRNA and have been involved in several pathological conditions including cancer. In fact, major hallmarks of cancer, such as the cell cycle, cell proliferation, survival and invasion are modulated by miRNAs. Cancer drug resistance (CDR) has also been described as being modulated by miRNAs. CDR remains a burden for cancer therapy and patients' outcome, often resulting in more aggressive tumours that tend to metastasize to distant organs. In this review we discuss the role of miRNAs influencing drug metabolism and drug influx/efflux, two important mechanisms of CDR.