I have serious doubts that you'd be able to see the 1 kDa difference at 65 kDa target on the SDS-PAGE (unless you'll make an ultra-extra-big one). I'd say, the only viable alternative is to do MALDI or other mass-spec opn the sample. <http://www.biowww.net/forum/read.php?f=6&i=925&t=871>