P-glycoprotein

http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/current_month.html

they pump toxins away from the cell, including cancer drugs, so they make them less useful, and this so cancers produce more P-glycoproteins to do just this.

so we are now trying to find ways to block P-glycoprotein from ejecting the drugs from the cells.