Figure 4.

Conditional activation of transgenes in pigs. A transposon containing a Cre-activatable transgene ('gene', red) interrupted by a selectable marker gene ('marker', green) can be used to obtain transgenic pigs with conditional expression of a transgene. The promoter driving expression of the marker/gene can be either ubiquitous (Ub) or tissue-specific (TSP), which would result in the ubiquitous or tissue-specific expression of the marker in F₀ pigs, as shown (green) to the left. Tissue-specific activation of the transgene can be accomplished in two ways: by crossing pigs that ubiquitously express the marker-interrupted transgene with a pig expressing Cre (blue) in a tissue-specific manner, or by crossing pigs that express the marker-interrupted transgene in a tissue-specific manner with a pig that ubiquitously expresses Cre. Controlled expression of the transgene or controlled excision of the marker allows expression of the transgene (to the right, red) in a specific tissue, shown here to represent the pancreas.