1. Department of Genome and Computational Biology, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. Centro de Biologia Molecular Severo Ochoa, CSIC-UAM, Cantoblanco 28049, Madrid, Spain.
3. Dipartimento di Genetica e Biologia Molecolare “Charles Darwin,” Universita “La Sapienza,” 00185 Roma, Italy.
4. Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.