The generation of transgenic mice by pronuclear microinjection and their subsequent breeding has been more efficient with F1 or F2 zygotes than with zygotes of inbred strains because inbred mice generally have a relatively poor reproductive performance (1). However, an inbred genetic background is preferable for genetic analyses such as the transfer of one allele of a mouse gene into a strain carrying a different allele (2). Likewise, for insertional mutagenesis experiments, inbred strains eliminate ambiguity caused by different genetic backgrounds and segregating markers in the progeny. As reported here, the inbred strain FVB/N is a good breeder with large litters, and the fertilized eggs of this strain have large prominent pronuclei, which facilitate microinjection of DNA.The ancestor of FVB/N is an outbred colony of Swiss mice N:GP (NIH general purpose mouse) established at the National Institutes of Health in 1935. From the N:GP colony, a second colony N:NIH (NIH mouse) was established in the early 1940s. In 1966, a project was begun to develop two populations of N:NIH mice. Mice were inoculated with pertussis vaccines, followed by a challenge with histamine diphosphate. Two strains were selected for sensitivity and resistance and were designated as histamine sensitivity factor sensitive (HSFS/N) and histamine sensitivity factor resistant (HSFR/N), respectively. In the early 1970s, a group of mice at the eighth inbred generation from HSFS/N line were determined to carry the Fv-lb allele for sensitivity to the B strain of Friend leukemia virus, in contrast to N:NIH mice, which were sensitive to the N strain of this virus (Fv-J ") (F.L., unpublished results). These mice were then inbred and offspring were selected for Fv-Jb homozygosity. To avoid confusion with the HSFS/N strain that is Fv-1 ", the Fv-J b strain was designated as FVB for Friend virus B-type susceptibility. This strain has been maintained since the late 1970s as an inbred strain without selection for either pertussis vaccine sensitivity or virus type. In this report, we provide a detailed characterization of the genetic background of the FVB/N strain and the advantages of using the strain to generate and study transgenic mice.MATERIALS AND METHODS Mice. FVB/N mice (F38) were obtained from the National Institutes of Health Animal Genetic Resource.Pronudear Measurement. Embryos were obtained by in vitro fertilization of superovulated oocytes as described (3).Embryos developing pronuclei between 6 and 7 hr postinsemination were cultured an additional 5-6 hr and photographs were taken with Nomarski optics. Pronuclear volumes were calculated from their diameters measured along the equatorial planes perpendicular to the location of the polar bodies and excluding the zonae pellucidae. Only embryos exhibiting both pronuclei were used for analysis.Generation of Transgenic Mice. Pronuclear microinjections were performed by standard techniques (1). Mice were maintained on a cycle of light from 6:00 a.m. to 8:00 p.m.Superovulation was induced by administra...

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