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.
Clark et al. Genome Biology 2007 8(Suppl 1):S13 doi:10.1186/gb-2007-8-s1-s13