Figure 4  Nested PCR strategy for genotyping of pre-implantation embryos. Due to the scarcity of extractable genetic material, a nested PCR strategy was developed in order to genotype pre-implantation embryos. Separate reactions were used to amplify a 429 bp fragment from the wild-type allele and a 389 bp segment from the mutant allele, both spanning the 3' end of the short (upstream) arm of the targeting vector. In both reactions, a 213 bp sequence located within the short arm was co-amplified with the 'diagnostic' products as an internal control. The same sense primers were used to amplify 'diagnostic' sequences in both reactions, whereas the antisense primers were allele-specific. The primers, represented by triangles, are depicted in two (nested) sets for each of the three reactions. Exons are represented by open, vertical rectangles, the PGK-Neo cassette by an open, horizontal rectangle, while thin vertical lines denote the boundaries of the short arm and the 5' end of the long (downstream) arm. A grey, horizontal rectangle delineates the segment of the wild-type allele that is replaced by the PGK-Neo cassette in the mutant allele.