ABCB1 and breast neoplasm: Wild-type mice were implanted orthotopically with breast tumors comprising three levels of Abcb1a/b expression and ABCB1 protein levels (Supplemental Fig. 2) found in clinical tumors [1]: basal, intermediate (3-fold increase in Abcb1a/b), or high (~ 521-fold increase in Abcb1a/b).