Genome Architecture, Gene Regulation, Stem Cells and Cancer Programme, Centre for Genomic Regulation (CRG), the Barcelona Institute of Science and Technology, Barcelona 08003, Spain Universitat Pompeu Fabra (UPF), Barcelona, Spain.|Genome Architecture, Gene Regulation, Stem Cells and Cancer Programme, Centre for Genomic Regulation (CRG), the Barcelona Institute of Science and Technology, Barcelona 08003, Spain Universitat Pompeu Fabra (UPF), Barcelona, Spain.