In previous studies, we demonstrated that the breast cancer resistance protein (BCRP, ABCG2) forms an S-S homodimer. The BCRP-C603S mutant substituting Ser for Cys-603 in the third extracellular domain formed both a 70-75-kDa monomer and 140-150-kDa dimer, suggesting that Cys-603 is an important residue in the covalent bridge. These results also suggested the involvement of other Cys residues in dimer formation. In the present study, we examined the possible involvement of the other extracellular Cys residues, Cys-592 and Cys-608, in the dimerization and transporter functions of BCRP using double and triple Cys-mutant BCRP transfectants. In SDS-PAGE under non-reducing conditions, BCRP-C592S.C603S and BCRP-C592S.C608S were detected as dimers whereas BCRP-C603S.C608S and BCRP-C592S.C603S.C608S were found only as monomers. This finding indicated that no Cys residues other than the three extracellular Cys are responsible for the dimer formation. The formation of BCRP-C592S.C603S dimer suggested the involvement of Cys-608 in the covalent linkage of this mutant BCRP. PA/C592S.C603S.C608S-cl.7 cells showed a significant level of multiple drug resistance and low-level accumulation of mitoxantrone. These results clearly demonstrate that BCRP functions as a drug resistance protein without covalent dimerization. Among drug-resistant Cys-mutant BCRP transfectants, PA/C603S, PA/C592S.C608S, and PA/C592S.C603S.C608S were found to be more resistant to the reversal effects of fumitremorgin C than PA/WT, suggesting some alteration in the substrate recognition in Cys-mutant BCRPs. In conclusion, Cys-mediated covalent dimerization is not required for BCRP to function as a transporter. In addition to Cys-603, Cys-608 may also be involved in BCRP dimer formation.