FIG. 1.

A gene trap event into the NS locus leads to a null NS mutation and early embryonic lethality in mice. (A) The structure of the retroviral construct along with the structures of the wild-type and mutated alleles and the positions of the oligonucleotides used for the PCR-based strategy are depicted. Exons are indicated by black boxes, and the long terminal repeats (LTR) of the viral vector are represented by light gray boxes. β-geo, lacZ/neomycin reporter/selection cassette. (B) PCR analysis of embryos with the indicated genotypes. The three primers, a, b, and c, are indicated. (C) Morphology of NS wild-type (+/+ or +/−) and homozygous mutant (−/−) E3.5 to E4.0 embryos. Magnification, ×200 (phase-contrast microscopy). (D) NS is expressed in the nucleoli of blastocysts. Blastocysts were recovered from NS^+/− intercrosses, fixed, and stained with anti-NS antibody (red). Nuclei were visualized with DAPI staining (blue). About 25% of the embryos failed to stain for NS. Magnification, ×200.