Design of Biological Systems, Systems Biology, Center for Genomic Regulation, Dr. Aiguader 88, 08003 Barcelona, Spain, Advanced Light Microscopy Unit, Center for Genomic Regulation, Dr. Aiguader 88, 08003 Barcelona, Spain, and Laboratory for Experimental Biophysics, Institute for Physics of Biological Systems, School of Basic Sciences, Swiss Federal School of Technology, 1015 Lausanne, Switzerland.