Summary.The multidrug resistance genemdr1, encoding P‐glycoprotein (P‐gp), can be expressed at high levels in tumour cells derived from normal tissues with constitutive high expression of this gene. In myelogenous leukaemia, the incidence of increased expression ofmdr1 gene contrasts with the low expression of this gene in normal bone marrow (b.m.). To detect cells expressingmdr1 gene in normal and post‐chemotherapy b.m., we usedin situRNA hybridization and RNA phenotyping by the polymerase chain reaction formdr1 mRNA detection. The presence of P‐gp was evaluated by immunocytochemistry with MRK16. Fifteen b.m. (eight normal and seven post chemotherapy) were tested byin situhybridization and either PCR (three b.m.) or immunocytochemistry (11 b.m.) or both (one b.m.). Within situmRNA hybridization, a subset (7.7×3.1) of b.m. cells expressedmdr1 mRNA in all cases tested, with no significant differences between normal b.m. and post chemotherapy b.m. 18 of myeloid recognizable cells and 7 of the cells with lymphoid morphology expressedmdr1 mRNA. By RNA phenotyping, the four samples tested for in situ hybridization and two additional post chemotherapy b.m. expressed mdr1. MRK16 was unable to detect a significant number of cells expressing P‐gp either by immunocytochemistry in the 12 b.m. tested forin situhybridization (0 in nine cases; 0.4, 1 and 3 of positive cells in three cases), or by flow cytometry in six additional normal b.m. (0–1.4 pos
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