Fig. 1. Gene trap integration into the C4st1 locus. (A) Schematic representation of the integration of the PT-1 gene trap vector into intron 1 of the mouse C4st1 locus and the formation of a C4st1-exonI-lacZ fusion transcript. (B) Embryos from intercrosses of C4st1^gt heterozygous animals were genotyped by Southern analysis and the ratio of lacZ to en-2 alleles measured. (C-F) Whole-mount in situ hybridization on E10.0 embryos using the indicated exon-specific C4st1 probes. In branchial arches (white arrows) and AER of limb buds (black arrowheads) in C4st1 mutant (gt/gt) embryos, a C4st1 exon I-specific signal (E) is present, but an exon II/III-specific signal (F) is absent.