The monitoring of hOCT1 mRNA expression in patients with chronic myelogenous leukemia (CML) was used for predicting the response to imatinib treatment. However, different cell populations from patients who received various degrees of pretreatment were used for this analysis. Therefore, several biases in the results and their interpretation may arise. We investigated hOCT1 mRNA expression in different cell populations of peripheral blood (PB) from healthy volunteers and in imatinib naïve de novo CML patients by analyzing changes in hOCT1 mRNA expression during the first 6 months of imatinib therapy. The hOCT1 mRNA expression was significantly higher in PB polymorphonuclears compared to mononuclears. The hOCT1 mRNA expression in total PB leukocytes is, therefore, preferentially determined by the percentage of polymorphonuclears. Expression in each analyzed group of cells was always significantly lower in imatinib naïve de novo CML patients compared to healthy volunteers. This difference disappeared after the initiation of imatinib therapy, suggesting that CML tumor burden and the degree of pretreatment at the time of monitoring were both influencing factors. Imatinib (IMA) uptake is predominantly mediated by human organic cation transporter 1 (hOCT1) [1] . Decreased expression or function of this protein can lead to a reduction of the intracellular concentration of IMA and, there-fore, lead to pharmacokinetic resistance to the drug [1, 2] . There are several data suggesting a correlation between hOCT1 activity/expression and the therapeutic outcome of CML patients treated with IMA [3] [4] [5] . Unfortunately, this issue has several uncertainties in the literature that may lead to the misinterpretation of previously reported data and the interpretation of future studies. First, hOCT1 activity, which is measured by a functional assay [1, 2] , must be clearly differentiated from mRNA expression monitoring [1, [3] [4] [5] . Second, the cell populations used for analysis vary widely in cell type, especially in studies that monitor mRNA hOCT1 expression [1, [3] [4] [5] . Furthermore, while some studies have monitored a well-defined group of patients with de novo chronic phase CML [3] , others make analyses on heterogeneous study populations that contain pretreated patients [4, 5] .