To investigate in vivo the functions of the phosphatidylserine receptor Ptdsr, we generated a null allele in the mouse by gene targeting (Figure 1a,1b,1c). In contrast to previously described Ptdsr-knockout mice [31,32], we used Bruce4 embryonic stem (ES) cells for gene targeting [33], thus generating a Ptdsr-null allele in a pure, isogenic C57BL/6J genetic background. The newly established knockout mouse line was named Ptdsrtm1Gbf (hereafter referred to as Ptdsr -/-). Heterozygous Ptdsr+/- mice were viable and fertile and showed no obvious abnormalities. Ptdsr +/- mice were intercrossed to generate homozygous Ptdsr-deficient mice. The absence of Ptdsr expression in Ptdsr -/- embryos was confirmed by RT-PCR (data not shown), and by northern and western blotting analyses (Figure 1d,e). Interbreeding of heterozygous mice showed that the mutation was lethal, since homozygous mutants were not detected in over 100 analyzed litters at weaning. To determine the stages of embryonic development affected by the Ptdsrtm1Gbf mutation, timed breedings were followed by PCR genotyping (Figure 1c) of embryos. We recovered fewer than the expected number of homozygous embryos from intercrosses of Ptdsr+/-mice. From a total of 1,031 embryos analyzed between gestational day (E) 9.5 and E18.5, 198 (19.2%) Ptdsr-deficient homozygous embryos were harvested, indicating that the introduced mutation is associated with a low rate of embryonic lethality in utero.