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The basic principle is called a reporter gene system

When putting genetic material(transfecting) into another organism be it bacteria, yeast or a pig you need to beable to quickly tell if the transfection has been sucessful

The simplest system is called the lacz reporter gene. This is a gene which will digest certain compounds and produce a blue colour. If you attached the lacz gene to the other genetic material you want to transfect then grow the bacteria on this compound you know if you have been sucessful because your colonies are blue

These genetic material from these pigs could potentially be used in a very similar manner