Resistance of cancer cells to chemotherapy is a serious problem for cancer treatment. Such resistance may develop during drug treatment or may be an inherent feature of a particular tumor type. The 170 kDa multidrug resistance 1 (MDR1) P-glycoprotein was the first human ATP-binding cassette (ABC) transporter to be identified and has the ability to mediate multidrug resistance in cancer cells. Multidrug resistance-associated proteins (MRP) including MRP1 and 2 are also involved in multidrug resistance in cancer chemotherapy. These proteins act as ATP-dependent molecular pumps that extrude a number of anticancer drugs from the cells, which results in decreases in their intracellular concentrations. [1][2][3][4] MRP1 was identified in the non-P-glycoprotein multidrugresistant small cell lung cancer cell line H69 after stepwise exposure to doxorubicin. 5,6) MRP1 shows relatively ubiquitous expression, whereas MRP2, which was initially identified as a canalicular organic anion transporter, is restricted mainly to the renal, intestinal and hepatic epithelia. Since there is extensive overlapping of the tissue distributions of Pglycoprotein and MRP2, it is likely that these two proteins play cooperative roles in pharmacological and toxicological protective functions, by mediating the efflux of different (but partially overlapping) sets of substrates. 7,8) P-glycoprotein extrudes large hydrophobic molecules that are uncharged or positively charged, while the members of the MRP family tend to pump out both hydrophobic uncharged molecules and water-soluble anionic compounds. The lung resistance protein (LRP), which is the major component of complex ribonucleoprotein particles called "vaults," was detected in a non-P-glycoprotein lung cancer cell line. 9) Like P-glycoprotein and MRP, LRP expression by tumor cell lines is believed to be associated with resistance to various anticancer drugs, 10) though the molecular function of LRP remains to be fully elucidated.Retinoblastomas are a rare intraocular malignancy occurring in early childhood. They occur in 1 in 15000 to 20000 live births irrespective of ethnicity or gender. The management of retinoblastomas is complex and includes enucleation and non-enucleation (conservative) techniques. 11) Retinoblastomas have traditionally been treated by enucleation of the affected eye and/or a combination of radiation therapy, cryotherapy or focal laser therapy. 11) Though external beam radiotherapy has been a standard treatment for retinoblastomas, it increases the risk of cosmetic deformities and secondary malignant tumors in the field of irradiation. Currently, the most popular conservative technique is primary systemic chemotherapy, which is also called chemoreduction. 12) Chan et al. 13) reported that increased expression of P-glycoprotein was detected in multidrug-resistant retinoblastoma cell lines as well as in the tumors from which the cell lines were derived, while drug-sensitive retinoblastoma cell lines were negative for P-glycoprotein staining. Subsequently, clinical studies...