PCR primers should be designed to be transgene-specific, usually
spanning a novel promoter-gene junction.
Southern blot digestions should be similarly designed to give a
transgene-specific band, distinguishable from possible endogenous bands
if the gene is mouse or mammal-derived.
In this example, the upper bands are endogenous mouse
bands, and the lower band is transgene-specific. Note that the
endogenous bands are single-copy in intensity, and the transgene band
increases in intensity in proportion to how many copies are spiked into
the mouse genomic DNA.