Figure 2

Selection cassette recycling of selectable marker in pigs. A transposon containing two genes, a transgene of interest ('gene', red) and a selectable marker ('marker', green) can be used to construct a transgenic pig. The promoters can differ for the two genes. Generally, the marker will be driven by a ubiquitous promoter (Ub), allowing selection for expressing donor cells (somatic cell nuclear transfer [SCNT]) or piglets (pronuclear injection [PNI] or lentiviral transduction [LVT]), whereas the promoter for the transgene could be a ubiquitous promoter or tissue-specific promoter (TSP). In the diagram the 'gene' is driven by a TSP. Expression in an F₀ animal is depicted by the pig on the left with the marker being expressed ubiquitously and the transgene being expressed in a tissue-specific manner, shown here as pancreatic expression. Crossing this pig to a pig that ubiquitously expresses Cre recombinase (blue) would result in F₁ progeny that lost expression of the ubiquitous marker and retained expression of the transgene in the pancreas. Ubiquitous Cre expression would occur in the F₁ (50% or 100%, depending on whether the Cre pig was heterozygous or homozygous), but this would be irrelevant to analysis of the F₁ phenotype. RRS, recombinase recognition site.