Chen’s interests have recently shifted to ABC transporters involved in the immune system and disease. Her work on two such transporters, P-glycoprotein and MRP1, has led to new insights into a mechanism by which some cancer cells mount resistance to chemotherapy. Discovered in the 1970s, P-glycoprotein recognizes an array of structurally related compounds and pumps them out of the cell. It plays a Jekyll-and-Hyde role in human health: When the cell in question is cancerous and the compounds are therapies targeting some aspect of the cell’s internal machinery, P-glycoprotein’s action reduces the effectiveness of chemotherapy. But, by doing essentially the same thing, P-glycoprotein also helps maintain the integrity of the blood–brain barrier by keeping certain types of molecules from entering the central nervous system.