Northern blot analysis of RFC mRNA levels in parental CCRF-CEM cells and their PT523-resistant sublines. Total RNA (30 μg, except for RFC overexpressing CEM-7A cells, which contained only 6 μg) from WT cells and the various PT523-resistant sublines was fractionated by electrophoresis on 1.2% agarose gels containing formaldehyde, blotted onto Zeta-Probe-GT nylon membrane (Bio-Rad) and probed with a [³²P]oligolabeled full-length RFC cDNA13 (A). The blots were then washed under high stringency conditions and exposed to x-ray films. The intensity of the 3-kb RFC transcript was determined by scanning densitometry and normalized to the methylene blue staining of the 18S and 28S rRNA bands (B). The ratio of the intensity of the RFC transcript in each cell line versus WT cells is depicted in the bottom of panel A.