Some introductory information about MDR1, the protein that mediates resistance to chemotherapeutic agents, from Wikipedia:
P-glycoprotein (plasma gycoprotein, abbreviated as P-gp or Pgp) is a well-characterized ABC-transporter of the MDR/TAP subfamily. P-gp is also called ABCB1, ATP-binding cassette sub-family B member 1, MDR1, and PGY1. P-glycoprotein has also recently been designated CD243 (cluster of differentiation 243). In humans, P-glycoprotein is encoded by the ABCB1 gene.
Pgp is extensively distributed and expressed in the intestinal epithelium, hepatocytes, renal proximal tubular cells, and capillary endothelial cells comprising the blood-brain barrier.
The animation below, produced by CancerQuest, shows how the MDR protein ejects drugs once they have entered the cell.