Although your high-school biology teach-er might not have told you, cells are like nightclubs. They have to make sure that the right individuals come in and that the bad individuals—those who might disrupt the normal activity inside the club—stay out. To that end, both nightclubs and cells employ “bouncers” to prevent unwanted characters from entering. For nightclubs the undesirable characters could be drunks; for cells the undesirable characters are noxious chemicals. And, as in a nightclub, the job of the cellular bouncer is to prevent any bad players from entering and to throw them out if they should happen to get in.

The cellular bouncers are efflux transporter pro teins belonging to the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily, one of the largest families encoded in the human genome (1). They sit in the membrane of cells, possess a common ATP-binding region referred to as the ABC cassette