Disruption of the Ctcf gene in 6B72 cells. DNA sequences flanking the U3NeoSV1 provirus in 6B72 cells were cloned and sequenced. The flanking sequences were identical to sequences in the rat genome, placing the provirus in the first intron of the Ctcf gene (A). Filled and open boxes indicate coding and non-coding exons, respectively. Flanking sequences 5' of the U3NeoSV1 provirus (B) include the first exon (shaded) of the gene.