Caveat

The use of sequence similarity is extremely powerful, and its importance is growing. However, there is no one-one correspondence between sequence and structure or sequence and function, because the converse of the first fact is not true. That is, high sequence similarity usually implies significant structural or functional similarity (the first fact), but structural or functional similarity does not necessarily imply sequence similarity. On the topic of protein structure, F. Cohen [#!COH!#] writes ``... similar sequences yield similar structures, but quite distinct sequences can produce remarkably similar structures." This converse issue is one indication of the many open research areas even in sequence-oriented bioinformatics.