Unselected human cancer cell lines: Oligonucleotide hybridization of PCR-amplified DNA and RNA from unselected human cancer cell lines is shown on the left and right of the figure, respectively. The results are quantified as in Fig 3. Cell lines homozygous for one allele show expression of this allele exclusively. Most heterozygous cell lines have relative levels of expression approaching unity. KB 3-1 and MCF-7 express no MDR-1. HMC-3 and HMC-4 are control oligonucleotides slotted as internal controls as described in Fig 3.