I'm trying to work through the protocols for TUNEL staining using In situ Cell Death Detection Kit (Roche), and the recipe of the permeabilization solution keeps bugging me. I admit, we never used sodium citrate in other assays before, especially that of immunohistochemistry, tissue nor cell. We simply added Triton X-100 to PBS in both of these cases. I searched the web and I found a lot of recipes included these particular compounds, but nobody explained why it's there or what it's for. Can anybody shed some light on this matter?