ABCG2 and cancer: The most common mechanisms that produce drug resistance in cancer cells include: (1) altered cell cycle check points; (2) induction of emergency response genes; (3) alterations in membrane lipids; (4) compartmentalization; (5) inhibition of apoptosis; (6) altered drug targets; (7) decreased uptake and (8) increased efflux of drugs by ATP-binding cassette (ABC) transporters [3].