Pleiotropic drug resistance is a complex phenomenon that involves many proteins 
that together create a network. One of the common mechanisms of multidrug 
resistance in eukaryotic cells is the active efflux of a broad range of 
xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces 
cerevisiae is often used as a model to study such activity because of the 
functional and structural similarities of its ABC transporters to mammalian 
ones. Numerous ABC transporters are found in humans and some are associated with 
the resistance of tumors to chemotherapeutics. Efflux pump modulators that 
change the activity of ABC proteins are the most promising candidate drugs to 
overcome such resistance. These modulators can be chemically synthesized or 
isolated from natural sources (e.g., plant alkaloids) and might also be used in 
the treatment of fungal infections. There are several generations of synthetic 
modulators that differ in specificity, toxicity and effectiveness, and are often 
used for other clinical effects.