1. Department of Cell Biology and Physiology, Washington University School of Medicine, 600 South Euclid Avenue, Campus Box 8228, St. Louis, MO 63110, USA.
2. Department of Molecular Biology and Genetics, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA.
3. Howard Hughes Medical Institute.
4. Department of Bioinformatics, Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawińskiego 5a, 02-106 Warsaw, Poland.