Quantitative determination of the human MRP1 and MRP2 mRNA expression in FACS-sorted peripheral blood CD4 + , CD8 + , CD19 + , and CD56 + cells ATP-binding cassette (ABC) transporters play an important role in the transport of various endogenous and exogenous compounds across bio- logic membranes (1). In cancer cells, they are known to confer multidrug resistance through enhanced efflux of chemotherapeutics. The human multidrug resistance-associated protein 1 (MRP1) and 2 (MRP2) belong to subfamily C of the ABC trans- porters, and were first identified by Cole et al. (1992) (2) in human small cell lung cancer cells, and by Taniguchi et al. (1996) (3) in cisplatin-resistant human head and neck cancer cell line, respectively. Within hematopoietic cells it has been found that CD3 + cells exhibited the highest and CD19 + cells the lowest MRP1 mRNA levels (4). Protein analysis determined that CD4 + cells exhibited higher MRP1 expression than other cells. In another study, Abba- szadegan et al. (5) showed that regardless of the cell lineage, normal peripheral blood and bone marrow hematopoietic cells express similar basal levels of the MRP1 mRNA. Laupe` ze et al. (6) investigated MRP activity in normal mature leukocytes using the Oselin K, Mrozikiewicz PM, Pa¨hkla R, Roots I. Quantitative determination of the human MRP1 and MRP2 mRNA expression in FACS-sorted peripheral blood CD4 + , CD8 + , CD19 + , and CD56 + cells. Eur J Haematol 2003: 71: 119–123. Ó Blackwell Munksgaard 2003. Abstract: Objectives: ATP-binding cassette (ABC) transporters extrude a wide variety of endogenous and exogenous compounds. In cancer cells, they are known to confer multidrug resistance. The aim of the present study was to determine the expression of the multidrug resist- ance-associated protein 1 (MRP1) and 2 (MRP2), which are members of the subfamily C of the ABC transporters family, in human hemato- poietic cells. Methods: CD4 + , CD8 + , CD19 + , and CD56 + cells were isolated from whole blood by FACS-sort in 20 healthy volunteers. MRP1 and MRP2 mRNA levels were quantified using real-time reverse transcriptase–polymerase chain reaction (RT–PCR) assays on a Light- Cycler TM (Roche, Mannheim, Germany). Results: The MRP1 mRNA exhibited the highest abundance in CD4 + cells (7.4 · 10 3 ± 3.19 · 10 3 molecules/ng of total RNA), followed by CD8 + > CD19 + > CD56 + cells. The MRP2 mRNA expression was highest in CD4 + cells (6.7 · 10 2 ± 2.84 · 10 2 ), followed by CD8 + > CD56 + > CD19 + cells. No correlation between the MRP1 and MRP2 mRNA expression was observed. Interestingly, b2-microglobulin mRNA expression in CD19 + cells was found to be twofold lower in comparison with other cells. Conclusions: On an mRNA level both MRP1 and MRP2 were expressed in peripheral blood cells, with more than sevenfold higher MRP1 expression in all cell populations investigated. The impact of the MRP1 and MRP2 transcription in these cells remains to study. The use of b2-microglobulin as a housekeeping gene could have a critical impact on the interpretation of RT–PCR data. Kersti Oselin 1 , Przemyslaw M. Mrozikiewicz 2 , Rein Pähkla 1 , Ivar Roots 2 1 Institute of Pharmacology, Tartu University, Tartu, Estonia and 2 Institute of Clinical Pharmacology, University Medical Centre CharitØ, Humboldt University, Berlin, Germany Key words: ATP-binding cassette transporters; multidrug resistance-associated protein 1 and 2; human PBMC Correspondence: Kersti Oselin, Institute of Pharmacology, Tartu University, Ravila 19, 51014 Tartu, Estonia Tel: +37 27 374 350 Fax: +37 27 374 352 e-mail: kersti.oselin@ut.ee Accepted for publication 9 May 2003 Eur J Haematol 2003: 71: 119–123 Printed in UK. All rights reserved Copyright Ó Blackwell Munksgaard 2003 EUROPEAN JOURNAL OF HAEMATOLOGY ISSN 0902-4441 119