Stable and strong promoter? I need a mammalian promoter that will maintain stable expression through differentiation. I was originally planning to employ UbC for this specific project, however new information from a different experiment has come to light and now I need a promoter that has both extreme stability and high strength (as you know, UbC is renowned for it's very low expression rates). Do any of you have suggestions? Perhaps I should look into a hybrid promoter of sorts... Thank you, CDB

There seems to be quite an amount of resources to help OP.

Addgene has a page on topic with list and description of common promoters usable in variety of organisms: Plasmids 101: The Promoter Region

Experimental biophysics textbook mentions some useful promoters as well, or this review.

Surprisingly, there has been some quantitative analysis of promoter strength: Systematic Comparison of Constitutive Promoters and the Doxycycline-Inducible Promoter, as well as effort toward stronger synthetic (version of CMV) Synthetic design of strong promoters

My conclusion is that either viral, or CAG (based on chicken actin) promoters might be useful candidates. But one cannot expect ubiquitous and even expression in various tissues/cells types without fine-tuning expression cassette.