1. Centre for Genomic Regulation (CRG), Department of Quantitative Cell Biology, The Barcelona Institute of Science and Technology (BIST) 1 , Carrer del Dr. Aiguader 88, 08003 Barcelona , Spain
2. 2 Universitat Pompeu Fabra (UPF), Barcelona, Spain
3. 3 Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluis Companys 23, Barcelona 08010, Spain