Glad I was able to. Well the speed of making mRNA can be attributed to two major factors. One being the promoter and transcription factor of course. The second being the sequence if gene itself. The former decides the binding and availability of RNA pol to the gene and later decides speed at which RNA pol moves on the gene. Since RNA pol is available at sufficient amount in all cells. I doubt if cell type used will affect speed of mRNA synthesis. It can however effect processing if mRNA and its translation. But then this is what I know of , I am not aware of any study done to test how any other factor in nuclei of different cell type effects speed of RNA pol. Any I guess I digressed a bit. But yes for this paper they used HEK because they are smooth to work with then it comes to availability, culture and even in terms of inserting DNA for different genes.

I don't know if you saw my last blog, but I have a picture of HEK cells I inserted DNA inside to produces viruses containing GFP gene (along with some shRNA, but I will keep that secret for now).

Anyway , I don't think that HEK are best cells as far as using this method in humans is concerned. For starters they are transformed immortalized cells. They can grow even if you spit on them (I even tell that to students that come to me for training in the lab) . Moreover they produce all kind of crap that you may not want in your body. So if I was to treat humans with this method, HEK won't be my cells of choice.