Intrinsic and retinoic acid induced expression of MDR1 gene in parental and RARα transfected cells. k- – water. ATRA (5 μM) was added to cell cultures 24 h after seeding for 48 h. Then the cells were collected and processed as specified in "Methods" (RNA isolation and reverse transcriptase polymerase chain reaction (RT-PCR) analysis of MDR1 gene expression). The optimal numbers of PCR cycles were 33 for MDR1-specific product for all cells lines except KG1 and KG1/RAR, for these cells the numbers of PCR cycles for MDR1-specific product were 26. This figure is representative of 2 separate experiments.