title: A more efficient method to generate null mutants using Hprt-Cre with floxed alleles.
abstract: OBJECTIVE
  The generation of nonviable homozygous null mouse embryos from heterozygote null/+ breedings can be highly resource consuming, with only 25% of the embryos in the litter being null mutants. We hypothesized that (1) we could double the number of homozygous null mouse embryos in a litter without reducing litter size using Hypoxanthine-guanine phosphoribosyltransferase-Cre (Hprt)-Cre (which is active in the female germ line at the time of fertilization), and (2) these homozygous null mutants would be identical to mutants generated through traditional null/+ breedings.
  
  
  METHODS
  To test this hypothesis, we used a conditional allele Fgfr2IIIb(flox). This allele when recombined is identical to the Fgfr2IIIb(null) allele. An F1 generation of Fgfr2IIIb(rec/+); Hprt(Cre/+) females was created by mating Fgfr2IIIb(+/+); Hprt(cre)(/cre) females to a Fgfr2IIIb(flox/flox) male. The F1 females were then mated to a Fgfr2IIIb(flox/flox) male. F2 embryos were genotyped, and the morphology and histology of the lungs, intestine, limbs, and brain were analyzed.
  
  
  RESULTS
  The Hprt-Cre mating strategy results in 51% of pups being genotypic homozygous null embryos (85/166) vs 23% for the standard null/+ approach (38/167). These embryos did not express the Fgfr2IIIb transcript and were phenotypically identical to null embryos generated through standard null/+ breedings.
  
  
  CONCLUSIONS
  The Hprt-Cre mating strategy increases the number of homozygous mutant embryos in a litter without decreasing litter size. Embryos generated through this approach are phenotypically identical to those from standard heterozygous breedings. We recommend this approach to investigators using a model system that relies on the generation of homozygous null embryos.
authors.affiliation: Section of Pediatric Surgery, Department of Surgery, University of Wisconsin SMPH, Madison, WI 53792, USA. Nichol@surgery.wisc.edu
authors.forename: Peter F
authors.initials: PF
authors.lastname: Nichol
authors.forename: Robert
authors.initials: R
authors.lastname: Botham
authors.forename: Yukio
authors.initials: Y
authors.lastname: Saijoh
authors.forename: Amy L
authors.initials: AL
authors.lastname: Reeder
authors.forename: Krzyztoff M
authors.initials: KM
authors.lastname: Zaremba
mesh_terms: D000483:Alleles; D000818:Animals; D005260:Female; D006720:Homozygote; D007041:Hypoxanthine Phosphoribosyltransferase / Q000494:pharmacology*; D008297:Male; D051379:Mice / Q000235:genetics*; D009154:Mutation
chemical_list: D007041:Hypoxanthine Phosphoribosyltransferase