RT-PCR analysis of the levels of expression for MDR genes in the parental cell line INER-51 as well as the PSC-1 cell clone. a) Amplification of MDR-1 gene visualized by ethidium bromide (left) or autoradiagraph (middle). The expression of G3PDH gene was used as a constitutive control for the integrity of the RNA molecules. b) Amplification by RT-PCR of other non-MDR-1 genes related to MDR phenotype. As control (CTL) of gene expression, the next cell lines and tissues were used: INER-37 cell line for MRP1 and GST-μ; A427 cell line for MDR-1; HeLa cell line for topoisomerase I, topoisomerase IIα and topoisomerase IIβ; finally, placental tissue was used as a control of BCRP expression.