Embryos derived from Hprt-deficient ES cells by tetraploid aggregation. A-C) Photographs of embryos at E13.0 derived from F3 cells. Pigmentation can be seen in the eyes (arrow) reflecting their ES cell origin. D) Southern blot analyses of BamH1-digested genomic DNA isolated from five (#1 – #5) F3 ES cell-derived embryos that was hybridized to a radiolabeled intron 3 probe. Only the mutant allele is detected in F3 ES cell derived embryos. The wild type allele can be seen in female F3 offspring as discussed in the text.