Expression of MDR1 transcript in parental HL-60R, HL-60R/neo, and HL-60R/196Rz cells by RT-PCR analysis and subsequent Southern blotting. 196 MDR1 ribozyme was cloned into the expression vector, pHβAPr-1-neo and transfected into HL-60R cells by electroporation (HL-60R/196Rz). Control clone was derived by transfection of empty pHβAPr-1-neo alone (HL-60R/neo). A total of 1 μg of total RNA was reverse transcribed to cDNA and amplified by PCR with primers specific for MDR1 and β-actin as an internal control; the amplification products were electrophoresed, transferred, and hybridized with probes specific for MDR1 (upper bands) and β-actin (lower bands) genes. Lane 1, HL-60R cells; lane 2, HL-60R/neo; lane 3, HL-60R/196Rz cells.