Before revision:  It is likely that comparison and alignment of biological networks will similarly revolutionize our understanding of biological systems . Existing network alignments are based largely upon information external to the networks, such as sequence, because  purely topological alignment of large, noisy networks is a theoretically challenging problem that has yet to be solved .
After revision:  It is likely that comparison and alignment of biological networks will similarly revolutionize our understanding of biological systems . Existing network alignments are based largely upon information external to the networks, such as sequence, because no good algorithm for purely topological alignment of large, noisy networks is a theoretically challenging problem that has yet to be solved .