No negative Genotypes!

[lciaquin]

hello all! I generated some transgenic mice with one randomly integrated transgene. I got 7 founders. I bred the "F line" to a non-transgenic mouse, this resulted in two litters (one with 4 pups, one with 5 pups of both sexes). All of the pups were positive for the transgene.
I bred the two of the pups to non-transgenics, and this time I have three litters with a total of 19 pups, that are all positive for the transgene!
I have another litter of 10 pups, which have not been genotyped yet.
By RT-PCR of 3 mice, they are positive for transgene RNA. By western blot of 3 mice they are positive for expression of the transgenic protein.
My question (obviously) is how can this be???? The other 6 lines bred to non-transgenics resulted in positives and negatives in normal mendelian ratios. I had negative controls for the PCR reactions. I have tried three different sets of genotyping primers.
I have some ideas of what could be happening, but I'm interested in some opinions, so I won't bias you with my ideas.
Thanks for any input!
Laura