when to set up a RT-PCR for mRNA expression? - detect transcription of transgene (Aug/10/2008 )

Can any body let me know, when is the right time to confirm that the transgene is getting transcribed (set up RT-PCR experimet)?

Since the litters of F1 generation needs to be genotyped, I can not know which pups are transgenic before 3 weeks, does that mean the mouse needs to be sacrificed once it is transgene positive (after 3 weeks) and take out the tissue for RNA isolation?

Is it neccesary to do RT-PCR with all the transgenic pups (different litters from different mouse colonies?

Thanks for your time!

Where and When is your TG expressed?

If it exists in every tissue there's no need to sac the mouse, just take some blood.