Just to emphasize how careful you have to be with all these probes and labels, consider what I'm hearing now from Remigiusz Serwa of the Tate group at Imperial College. His group is looking at farnesylation. People have tried making azido-containing substrates, for later "click" fluorescent labeling of proteins that pick up the label, but the azido group turns out to be a loser here. It's too polar in the greasy world of prenyl groups, and things go haywire.

You'd think that switching the click reaction around would be the answer here - make an alkyne group to be picked up by farnesyltransferase and you're in. But the ones that have been tried so far are terrible substrates for the enzymes. He seems to be on the way to solving that problem, but (interestingly) isn't revealing the structure (yet) of his probe. Must be a manuscript on the way - probably with a patent on the way before that?