I was speaking with a gradstudent studying string theory the other day, and he mentioned that currently "branes" higher than just strings are currently treated classically. As he is still learning, he couldn't comment further. My questions are this: 1) It is my understanding that the "prediction" of the number of dimensions comes from string theory only being finite/self-consistent for a certain set of dimensions, and they take the smallest of these as the prediction. Is it possible that when quantizing the higher dimensional branes, that this will restrict the set of "allowed" dimensions even more? Is it possible it will reduce this set to zero (ie. it can't be finite/self-consistent with quantized higher branes)? Or is there some mathematical theorem to protect it? 2) Are there technical issues preventing considering quantized branes? ...or is it technically straight-forward, but just very messy, so people focus elsewhere?