You want to make a transgenic mouse. The xyz gene is normally expressed in every cell of the embryo. You fuse the promoter (Pxyz, 300 bp) for the xyz gene to the DNA sequence encoding green fluorescent protein (GFP) (900 bp). You inject this fusion transgene into a single 1-cell mouse embryo.
............_________ Pxyz GFP_________...........
As the embryo grows, the xyz promoter should be expressed in every cell. However, your transgenic mouse does not express GFP. You are confident in your experimental method and believe that the construct did, in fact, integrate into the genome.
Question:
You amplify and sequence the integrated construct and flanking regions in both directions. On one side, your construct is flanked by a 5000-bp region of largely random sequence. On the other side, your construct is flanked by 200 "TTAGGG" repeats. Based on these results, propose a hypothesis as to why your construct is not expressed. You decide to reinject the same construct into another mouse embryo in an attempt to integrate the construct elsewhere in the genome. You are happy to find that this mouse does show GFP expression in every cell.