PGP and breast cancer: QDs, indeed, have potential in connection with MDR phenomena and they were tested with promising results also as blockers of ABC efflux transporters in human breast cancer SK-BR-3 cells [42], as P-gp protein expression down-regulators and apoptosis inducers in lymphoblastoid cells and BALB/c nude mice [43], and as inhibitors of P-gp drug efflux pumps, which was accompanied by overexpression of apoptosis-related caspase proteins in human hepatocarcinoma cells and nude mice [43].