(A) Schematic representation of the genomic structure and partial restriction map of the wt and targeted mouse Xpd loci. For the wt Xpd allele, shaded boxes represent coding regions of exons 12 and 19–23; the 3′UTR is represented by an open box. TGA indicates the translational stop codon; PolyA indicates the polyadenylation signal. For the XpdTTD targeted allele, the 194–base pair (bp) human XPD cDNA fragment fused to exon 22 is indicated as a striped box including the TTD (R722W) mutation indicated by a vertical arrow. Chicken β-globin exons 2 and 3 including the 3′UTR are indicated as black boxes with corresponding Roman numerals followed by the β-globin polyadenylation signal (PolyA*). For the Xpd†XP and Xpd†XPCS targeted alleles, vertical arrows indicate XPCS (G602D-encoding) and XP (R683W-encoding) mutations in exons 19 and 22, respectively. The unique 3′ probe located outside the targeting construct is marked by a thick black line. Restriction sites: B, BamHI; C, ClaI; E, EcoRI; H, HindIII; Hp, HpaI; Sf, SfiI.