National Center for Biotechnology Information NLM, NIH, Bethesda, MD, USA. wilbur@ncbi.nlm.nih.gov|National Center for Biotechnology Information NLM, NIH, Bethesda, MD, USA. wilbur@ncbi.nlm.nih.gov|National Center for Biotechnology Information NLM, NIH, Bethesda, MD, USA. wilbur@ncbi.nlm.nih.gov