Characterization of molecules with tightly controlled expression patterns during differentiation represents an approach to understanding regulation of hematopoietic stem cell commitment. The multidrug resistance-1 (MDR1) gene product, P-glycoprotein, and the breast cancer resistance protein (BCRP) are expressed differentially during hematopoiesis, with the highest levels in primitive bone marrow stem cell populations that are CD34^low and CD34⁻, respectively. Roles for ATP-binding cassette (ABC) transporter superfamily members in conferring drug resistance have been extensively described. However, recent hematopoietic overexpression studies have begun to reveal previously unknown roles for ABC transporter function in normal and malignant hematopoiesis. Expression of MDR1 and BCRP transporters in the myeloid lineage has been reported in blasts from acute myeloid leukemia, but very low to undetectable in normal myelomonocytic cells. Retroviral-mediated dysregulated expression of the MDR1 transporter resulted in increased hematopoietic repopulating activity and myeloproliferative disease in mice. A distinct functional role for the BCRP transporter as a negative regulator of hematopoietic repopulating activity has recently been demonstrated using the same approach. Additionally, the presence of BCRP expression specifically on hematopoietic side-population stem cells and neural stem/progenitors, makes BCRP an attractive candidate marker for isolation of stem cells with the ability to respond to diverse environmental cues. Regulation of stem cell biology by ABC transporters has emerged as an important new field of investigation. In light of these findings, it will be critical to further characterize this family of proteins in hematopoietic lineage-restricted stem cells and in pluripotent stem cells capable of crossing lineage barriers.