1. Centre for Genomic Regulation (CRG) , The Barcelona Institute for Science and Technology, Carrer del Doctor Aiguader, 88, 08003 Barcelona, Spain
2. Universitat Pompeu Fabra (UPF) , Barcelona, Spain
3. Section for Computational and RNA Biology , Department of Biology, University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen, Denmark