I`m running a western blot of lysates from cells overexpressing proteins with a ZZ-domain (part of protein A). I observe non-specific signal from my GAPDH-antibody from the same locations where I detect the protein (this is a fluorescence based western with incubation with both antibodies simultaneously). I was wondering if it is possible that the protein A tag will -despite denaturation and reduction- still bind any antibody. (My GAPDH antibody is from mouse). If this is the case does anyone have a suggestion how to prevent this?