I had a chance today to be asked about whether there were any good inhibitors for a particular class of protein (unfortunately, my answer was "No, not that I've ever seen"). And it got me to thinking: what are the classes of proteins for which that applies? There are plenty of examples on the positive side of the question. There are of course tons of different kinase inhibitors of varying levels of selectivity, and if you needed to find one for some as-yet-unconquered human example, you could feel fairly good about your chances, depending on how selective you needed your final compound to be. There are quite a few serine and cysteine protease inhibitors out there, too, again with varying selectivity, and while you're not just going to stroll over and find a great compound lying on the ground, you could certainly start a project against a new member of these classes without people laughing at you. PDE enzymes, topoisomerases, hydrolases, cyclooxygenases, histone deacetylases, reductases, DNA gyrases, several of the nuclear hormone receptors - there are several other classes with a list of known inhibitors against them, and although some of these are rather-wide ranging and thus come with no guarantee of success, you at least know that the mechanisms have been more or less successfully targeted in the past.