RNase protection assay of normal PB cells and AML patients. In this experiment, the MDR1 antisense probe was used. The KB cells were used as a negative control and the multidrug resistant KB-C1 cells were used as a positive control. The sizes of protected products were indicated by arrows. Radioactivity corresponding to the 130/134-bp bands and mRNA levels obtained by quantitative RT-PCR analysis are presented below the panel.