For instance, where the transgene is inserted, if it's downstream of an enhancer that's muscle specific, then you can get a, Mao said it overexpresses sirtuins just in muscle.
Researchers learned to control the expression of transgenes, and were soon racing—in both academic and industrial contexts—to create organisms capable of expressing human genes for the production of human hormones.
So we're talking about it when Peter says transgenic mouse, he's talking about a mouse that has this transgene that causes it to express more sirtuin-6 than it ordinarily would.