Department of Biochemistry and Biophysics, UCSF, 1700 4th Street, San Francisco, CA 94143-2542, USA. rwu@genome.ucsf.edu|Department of Biochemistry and Biophysics, UCSF, 1700 4th Street, San Francisco, CA 94143-2542, USA. rwu@genome.ucsf.edu|Department of Biochemistry and Biophysics, UCSF, 1700 4th Street, San Francisco, CA 94143-2542, USA. rwu@genome.ucsf.edu|Department of Biochemistry and Biophysics, UCSF, 1700 4th Street, San Francisco, CA 94143-2542, USA. rwu@genome.ucsf.edu|Department of Biochemistry and Biophysics, UCSF, 1700 4th Street, San Francisco, CA 94143-2542, USA. rwu@genome.ucsf.edu