ATP-binding cassette (ABC) transporters are a family of proteins that mediate multi-drug     resistance (MDR) via ATP-dependent drug efflux pumps.  Abnormally expression and function would result in tumor MDR.  That is the most important mechanism of MDR.  The inhibition of ABC transporters as a   strategy to reverse MDR in cancer has been studied extensively.  In this review, we reviewed the structure and function of ABC transporters, and focused on the research advances in the mechanism of tumors MDR mediated by ABC transporters and the development of their modulators and reversal strategies.