National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, 8600 Rockville Pike, Bethesda, MD, USA. gang.fu@nih.gov.|School of Informatics & Computing, Indiana University, 107 S. Indiana Ave, Bloomington, IN, USA.|School of Informatics & Computing, Indiana University, 107 S. Indiana Ave, Bloomington, IN, USA.|Department of Medicine, Stanford University, 450 Serra Mall, Stanford, CA, USA.|College of Computer and Information Science, Northeastern University, 360 Huntington Avenue, Boston, MA, USA.|National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, 8600 Rockville Pike, Bethesda, MD, USA.