Targeted disruption of HPRT locus. (A) Maps of the targeting constructs, bovine HPRT locus and structure of targeted locus. Locations of HPRT exons, pGK-puro cassette, nls, PCR primer sites, PCR product size and end sequencing regions are shown. (B) Sequence of nls, two 72 bp tandem repeats from SV40 enhancer. (C) Number of random and targeted insertions obtained with each targeting vector per 9.5 × 106 cells. (D) Diagnostic PCR of four 8-AG-resistant colonies. (E) End sequence of PCR product. PCR primer sites are shown in italics, exon 4 and exon 6 sequences are shown in bold.