1. Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology , Barcelona 08003, Spain
2. Department of Biochemistry and Molecular Biology, University of Chicago , Chicago , IL 60637, USA
3. Department of Chemistry, University of Chicago , Chicago , IL 60637, USA
4. IEO European Institute of Oncology IRCCS, Department of Experimental Oncology , Milan 20139, Italy
5. Universitat Pompeu Fabra (UPF) , Barcelona 08002, Spain
6. ICREA, Pg. Lluís Companys 23 , Barcelona 08010 , Spain