The recent identification of mutations at arginine 482 (R482) in human Breast Cancer Resistance Protein (BCRP) in two drug-selected cell lines largely explains some discrepancies observed in the cross-resistance profiles of human cell lines overexpressing this multidrug transporter. We find that each of three mouse cell lines independently selected for resistance to the anthracycline doxorubicin also acquired mutations in the cognate mouse transporter Bcrp1 exclusively at R482. Although the mouse Bcrp1 amino acid substitutions (M or S) are distinct from those seen in the human cell lines (G or T), they all have similar consequences: (a) greater resistance to anthracyclines (and bisantrene); (b) relatively lower resistance to topotecan; (c) greatly enhanced efflux of the dye rhodamine 123. The ready selection of R482X mutations seen in vitro might also occur in tumors treated with anthracyclines. Thus, it is noteworthy that the efficacy of Bcrp1 inhibitors applicable in vivo was not markedly affected by the presence of the mutations. We found that the Bcrp1 mutations all occurred after previous amplification and overexpression of the wild-type gene under doxorubicin selection; wild-type Bcrp1 is evidently able to mediate substantial resistance to anthracyclines, and this was confirmed in Bcrp1-transduced cell lines. These observations emphasize the general importance of the arginine at amino acid 482 for substrate specificity of the transporter, while reminding us that unmutated Bcrp1 remains a potential source of resistance to anthracyclines and a potential factor in anthracycline pharmacokinetics. The same is most likely true of human BCRP, given its profound similarities to mouse Bcrp1.